This will be a short review.

When did Katar tell Koursa his father was a doctor? If that happened then I forgot. And correct me if I'm wrong but the father was a soldier, right? Perhaps a medic, I remember he had medical books which he gave to Katar for some sort of self defense idea in a very odd scene.

"warm-skin act" I cannot understand what you mean here.

The bone-setting scene lasts for quite a long time and is a rather unpleasant experience. Now this could be what you are going for but I'm just letting you know my reaction.

Slithersslivers

I like the truth lies game.

The fight with Kira is quite good and nicely done. Also, the description of the Avatar state at the end is properly unsettling and unnerving. This is a good chapter for wrapping up the action of the last few sections in a bit dramatic blast. However, this chapter was a little rougher on editing errors like it didn't get as many once overs as the others usually do. Lots of typos and it's/its errors, many of which I didn't get around to noting here.

"he's been watching Katar" should be "he'd" to preserve the past tense you've been telling the story in.

"Katar laid" should be "lay". Laid is the action of setting down, not the continuous state.

"his eyes weren't even open and he didn't even speak..." I think you can take out one of these "even"s

Huh, gold light from the scar. Usually Avatar stuff is accompanied by the blue/white light of Raava. Interesting.

"Similar lights wondered behind shut eyelids." I would consider revisiting the word-choice of "wondered". I think I get what you are going for but it sounds a little off.

In the third paragraph I think the first comma and period can be switched.

"Katar came exhausted" "became"

You describe the shape of the cuffs twice in the same paragraph. You can take out one.

Interesting introduction of earthbending glass. The description of the mental process of treating glass like a slow moving liquid caught my eye. I thought that the whole "glass is technically a liquid" thing was a popular myth. The idea was that people looked at old glass panes and saw that the glass was thicker at the bottom. However, the truth was that in the old days glassmaking was less precise so panes were usually made slightly unevenly and installers put the thicker side down out of simple structural instincts. I'll admit that I'm faintly remembering something I heard or read long ago but this is what was on my mind.

On another unrelated note, I never really liked how the show talked about "metalbending", setting the tone for calling "lightningbending" and "bloodbending" and such. Bending metal is still earthbending, just controlling the earth within the metal, and in Avatar world electricity is just another form of fire like ice is another form of water. However, this is the MOST PETTY of niggles so completely ignore me. Thus was a tangent.

"As the sun hid beneath the shade, so did his mood darken." I'm not sure this simile works. After all, the sun is above the shade by definition, right?

When you switch back to Koursa's perspective, the first "she" should be replaced with her name so we know whose eyes we're using.

"A flash of light stroke there..." I don't think stroke is the right word.

The transition to Nite and Sumi fighting seems a little abrupt. Perhaps throw in a bit more description of the boiling aggression before you switch to Koursa's little section. Like ending with Nite screaming in frustration or something.

You say Katar is remembering "Everything has an inner rhythm". Maybe mention what he is remembering that from?

"One cannot feel sorry for lost foxhounds so strongly that he can bend the earth." What?

You have a neat tack here of focusing on the psychological aspects of Bending much more than the shows did. Even when they tackled the mental aspects they were always strongly rooted in physicality. Air bending in the control of bodily urges, earth bending in the rejection of hesitant reflexes, Fire in the movement of chi through the body. You have a different approach which makes it new.

"There's an obstacle..." "There was an obstacle"

"It's shreikly gnarr" To start, it'sits but as a larger point this bit sounds very Lewis Carrol, which is fine in the right circumstances. Now I am not one to criticize making up words as needs be, but you haven't being doing it much up till now so these two back to back stands out.

"She moaned at him" I'm not sure "moaned" is an aggressive enough word for what's happening here. Moan has sad or lustful connotations. Growled, grunted, or snarled might be better.

In a few places here you slip into the present tense. Be careful of that.

Katar's quite good at earthbending for not having any instruction. Katara couldn't even make things go the right direction until she found the scroll. Of course, he probably has Avatar help.

"He herd her..." heard

"As her heals..." heels

I forget, why does Nite think Katar is a waterbender? Katar grew up in the earth kingdom, right?

There's twice around here where you have "thought" when you mean "though".

The paragraph that begins "Prompted, Katar's mind..." is very rough with errors.

All right, this chapter things moving forward even if it doesn't exactly deliver on explanations yet. I didn't get an explanation of how Rohan could make someone else the Avatar when he should currently be a reborn infant but I expect that answers are coming. You chose the spirit world setting from Seige of the North which, while I'll admit I'd expected you to take the chance to make up another new setting, was easily recognizable. Good atmosphere and I can feel the drumbeat of a unifying plot picking up.

Interesting, so you have people not believing in spirits here. Every other time in the series we've generally seen people treat them as a rare but real part of life. An interesting approach to take for Katar, especially since he was close with the Avatar for years. Not criticizing, just genuinely keen to see what you're going to do with this attitude.

"almost phased out." phased should be "passed"

Correct me if I'm wrong, but "walking animation" comes from video games and so feels rather anachronistic.

Talking about koursa's legs sinking up her torso sounds very odd. I'm guessing you are going for some evocative image of tiredness but legs moving up the torso is not something I would ever associate with that.

I'm assuming you named Jian after the word for sword, which is fitting (well, I'm assuming it's that jian. In english letters I can't tell what character you were going for.) If not it's a happy coincidence.

Is "pompous" really the word you want to use here? I'm not sure it fits.

"Short water" should be "shallow water"

"Crippled space magic" I am not sure what to make of this phrase. If anything calling something "space magic" feels odd in the setting, and I don't know why you threw in "crippled".

Alright, this chapter is a good little foray into some Western motifs. You did a nice job making Sumi's confinement feel uncomfortable. Tense cooperation always makes for interesting content. Now on to notes:

I have never heard the expression "cupid's bow" to refer to some type of anatomy. It may be a regional phrase so I'm just letting you know about a possible audience problem. It also brings in a few extra issues with referencing a mythological figure that doesn't exist in this world.

Ok, a few paragraphs in and now I am confused by how Sumi is confined. I had assumed she was buried standing up all the way to her neck, as you see in wild west films. But then it said she could look down and see her body, like she was laying horizontal and just covered by a thin layer of "soil" which if so wouldn't restrain her. So I am uncertain.

You do a good job conveying the heat mirage of someone approaching over flat ground.

Sun-bear-puma parses oddly. I know what you mean but it still looks like a strange three part hybrid. Maybe it could just be a puma-bear and the descriptions make clear its based on a Sun Bear? Or it can just be fine the way it is. Or mabey Sun Bearpuma? Just thoughts.

Be careful alternating between talking about Kira and including speech from Sumi. A bit where Sumi talks resets who "she" refers to. Also you say Sumi "asked in capital letters" but you don't actually use any capital letters in that previous sentence.

I'd think Sun bears would be better at digging. Don't they routinely dig up insect mounds or am i thinking of another species?

It sounds odd to call the Kai Krew "aliens" even if I know what you mean.

Is Wanshitong's library widely enough known in this era to be used as a common expression? It seemed more secret and mystical than that in ATLA. Of course, you time period is rather ambiguous in this story so maybe this is way in the past when the spirit was more welcoming? Still, we haven't really heard about any spirit interaction.

"Kira, here, can track them." I'm not sure any of those commas are needed.

"She talked him down, before turning to where the decision really lied." This sentence is poorly phrased. Also, that last word should be "lay".

You've got a lot of very inventive and creative action here. Both bounty hunters have neat fighting styles and you have good movement around your set. However, this chapter feels like it might have been written more quickly or with fewer revisions than your previous chapters. The quality of your grammar takes a little slip here, compared to your normal quality. I included the ones that jumped out at me most but the rest could benefit from another pass.

You might want to clarify where "up here" is in the first paragraph. I know that we haven't moved since the last chapter but occasionally there are time jumps between chapters and it's good form to orient readers at the beginning.

Ok, just letting you know I got mixed up at first and thought Nite was the woman since that was who we focused on most last chapter. Then I saw strongman and thought you were talking about someone else. This kind of things happen when I take an almost week break between reading chapters.

I'm not sure "Was his...presence so mere" Is a valid phrase. I think that "mere" doesn't fit here, at least I have not ever heard it used that way before.

Oh, wait. Katar recognizes these people? Interesting. Did you already mention that or is this the first time?

I don't really understand the exchange around "You have my name and my oath" What does Nite want Katar to do?

Ok, when you have the little bomb "drift" in I thought at first it was literally hovering slowly. I didn't get that it was supposed to be a slowmo shot until I read quite a bit more. Also, I didn't get or didn't remember that Nite has a prosthetic limb. I'm assuming it works something like Ming Hua but with earth?

That sounds like a BIG explosion. What was in that little bomb? (I was envisioning something the size of a large coin, perhaps I was wrong)

"Like his father thought him," I think that should be "taught"

"Consuming it fully." Sentence fragment.

Sphincter, while anatomically correct the way you use it, in colloquial english nearly exclusively refers to the anus. Using it here calls up images other than choking.

"a sharper and a sharper angle to..." delete second "a"

You know, we're very deep into the story for you to have not really explained what the connection between Katar and Kai is. I really would have thought that would have been explained six chapters ago or thereabouts.

Woah, your version of Avatarworld has cannons? Thats a big deviation to slip in super casually like this. Again, I sound like a broken record but the general technology you describe sounds more like the late 100 year war more than hundreds and hundreds of years previously. Perhaps it would be easier to come out front with this being an alternate history of some sort where Aang was killed during the genocide?

I don't understand what the "Stale" repetition means? I can not figure out what that word would mean in this context.

Very good job with the imagery and atmosphere at the beginning of the train ride. The argument that follows is nicely heated and maintains the energy well. The twin superstition is interesting although a little ironic since if I'm remembering right, even Han has seen enough to know that a lot of their troubles are very blatantly caused by Katar's presence, not Kai's. Then a sudden attack of Plot!

It seems odd that they'd have trains this far in the past. We didn't even see any in the fire nation during ATLA although I'd be fine with assuming they were somewhere off screen. It seems a bit ridiculous they'd have tanks and the giant drill but no trains.

I like "the train slouched forward"

It's interesting that you have created a history of previous Fire Nation settlement in the Hundred Year war colony lands, indirectly strengthening Sozin's case for justification occupying those areas. Not a criticism, just interesting.

"The words stopped his hand midway." I think maybe this should be "Her words..." As it is it is a little ambiguous if he is speaking or she is.

"...secluded form each other." Typo

That very heated confrontation is happening right in the middle of a very crowded carriage and it is a bit odd that you don't note reaction or involvement from any of the other people crammed in there.

"What are even on about?" Typo

Ok, this is another picky little detail, but you describe one clothing article as a "black tank top" That name just has a very modern connotation in my mind. Perhaps it could be a "black sleeveless shirt"? Like I said, I know it's a very minor hangup.

Kai repeatedly wishing he was a firebender so he could burn off the mark seems odd. I mean, besides the obvious ideas of tatoos, he could still burn himself if he wanted to. He has access to fire even if he can't make it with Ki magic. I don't know, that just struck me.

Ok, in the middle of the last fight you start referring to the previously ungendered " assailant" as "she" and it took me a bit to figure out "she" was not Koursa. I know that bit of action is a little deliberately confusing but you still might want to look over the section again.

The goofy conversation between Sumi and Nite was really jarring with the previously established tone but I think you may have been going for that.

This chapter has good intensity of emotion and interesting story content. You do a good job of capturing Katar's grief and the unearthly creepiness of the Avatar state. The extended sequence of the search for Katar holds up the anxiety well. As you go along in these chapters your writing becomes increasingly poetic. Perhaps a bit over-poetic by this point but I'm sure I do not have room to judge there. After all, you've read my writing.

Remember, anything I don't mention below is good.

In the second sentence you throw in a narrative "you" which doesn't fit with your previous style.
You say "slopped" instead of "sloped".
4th sentence "it's" instead of "its".
Friendly Cuofeng anachronism report: The matches are odd too. Even ATLA didn't have them, they used various versions of flint and steel (Except for maybe the mechanist? I forgot if he had them but then he would have invented them himself.)

Interesting way to slip in Katar and Rohan's romantic relationship. If you placed hints to that before I did not catch them so this casual mention surprised me. Also, I don't know what redstone is. I have only heard that name as a fictional material in a videogame.

Is there a reason you go so long without explaining what this mystery room is? At one point there is suddenly a shrine, but I am still confused as to where he is. (Wait, is this his old house? Did something happen to his mother that I forgot?)

Ah, so the burn on his hand has a considerably more mundane explanation than I was thinking. It could have been my own incorrect interpretation but I thought you were suggesting some profound mystical wound (like harry potter's scar or something) other than just an accident with fire and an unaccustomed wielder.

Again I could be wrong, but I think this chapter is the first time you ever mentioned Katar's father. He kind of comes out of nowhere to suddenly be mentioned a lot.

When you switch back the the present there is no indication of when or where we are and who Katar is talking to. The flashback jumped around in time enough that you need to reground us.

Ok, a title drop in there. Presumably a metaphor for Katar's own mental state? Perhaps it will become clearer a little later.

"Hopefully, they won't speak no more. Hopefully, she won't have to drop him." The first sentence is improper grammar and I don't understand what the second means.

You have a few uncapitalized "i"s scattered through this chapter.

"I plan on that," does not seem to grammatically follow the conversation. As it is, I am unclear on what you mean.

All right, a chapter of answers. The scene describing the transfer is nicely dramatic and visual even if the full explanation clashes a bit with my internal perception of what "the Avatar" is. But that's just a personal issue, the scene still works. This certainly explains why Katar is running, if it leaves me a little confused about where Kai comes in. And then there's my big moment of confusion you'll read about in a moment. I'm perfectly willing to admit the error could all be on my end and if so feel free to ignore and just chuckle at my stupidity. Now on to notes:

I suggest using Koursa's name instead of that first "she". Establish who we are talking about.

"When she lost back home she still had a home to go back to..." This is grammatically fine but just sounds a little repetitive to me. Consider rewording it slightly if you want.

You keep on putting emotional weight on Katar's swords but since we don't know what that's all about it sort of falls flat. Mystery is fine to make us feel intrigued but doesn't allow us to empathize with an emotion. Again, just my opinion. I know I'm guilty of this from time to time as well.

I had completely forgotten that Koursa just wanted to get across the wall and then was set to leave. In fact, I've forgotten why she wanted to get across the wall at all. You might want to throw some mention of that back in the chapters where they first saw the wall to refresh it in our memories.

I think the first sentence of the Katar section is missing some punctuation.

I think Haifu is talking to Katar's mother but the way it is written makes it sound like his own mother. On a similar note, in the next paragraph you switch to "he" referring to Katar but grammatically it would refer to Haifu. Careful with these pronouns.

"Hai-fu thought him the way of the sword..." Typo

Ok, maybe I missed something very obvious or forgot an important detail but I was very surprised when you slipped in that Rohan was the Avatar. I had assumed the old master Haifu was the Avatar so this very casual sentence about Rohan caught me off guard. You must hve mentioned Avatar Rohan in an earlier chapter but if you did I didn't notice the name matching. This could just be my poor reading but in any case, sparing no words for Katar's surprise at the revelation seems very odd.

I don't know what you are referring to about a medic saving a life. Is this something we've heard about before? Also you say "letters of appraisal" and I'm not sure appraisal was the right word.

"Stomped his fist heavenly" I have no idea what this means.

"terrify to the top of his forehead" This is a very strange expression. Or at least unfamiliar to me.

In the narrative Rohan's eyes are suddenly glowing without any fanfare or introduction. That's a scary sight, you might want to put a little more emphasis there.

Good description of ego death from Katar's perspective.

"Felt a cram surge up..." You keep using cram here as a noun. I only know it as a verb so I'm not sure what you're saying.

I like the idea of the Earth Kingdom's secret weapon even if I don't immediately get how they are going to use it. The whole scene with the great wave makes for good action, if the slightest bit repetitive of the previous scene. I like Katar saving Nite and vice versa. I am now wondering about Nite's goal if it sounds like he is not actually out to kill Katar for the bounty. I am also interested in Katar's relationship with the Avatar gestalt, however he ended up with it bound to his body. You keep teasing those scars and I keep not knowing what their significance is but I trust you'll tell me soon. Now on to the stream of consciousness comments:

I'm guessing the first word in the fourth sentence is supposed to be "he'd" instead of just "he".

The underwater base at their technology level is fairly preposterous but this is a world with magic and what is magic for if not building cool underwater bases out of stone and clay. I'm also surprised they know about tectonic plates already but I can just guess that something about their interaction with earth-bending might have sped up the discovery. Ignore my quibbling.

Ok, so the "weapon" is a technique to trigger fault lines for on-demand earthquakes. I'm not sure why that involves a hum but it could be some sort of harmonic resonance? That sounds scifi-magic enough to work although I'm not sure why it would. Still, an earthquake technique to that requires you being at ground zero in a rickety pre-industrial underwater base does not sound like the best idea.

On the whole subject of time period in general: I forget if you told me but when is this? It's before the hundred year war, but there are mechanical clocks so it shouldn't be very long before, several centuries at most. The problem with that is that we know the names of the four Avatars Avatars before Aang and even if you wave away the bulk of the Kyoshi age mixup and give Koruk an early death that still covers up to 300 years. Or some exceptional advances were made in machining very early and I am overthinking this whole thing because of a clock.

Kai is surprised about second earthquake in a day. I am not sure how much he knows about quakes but multiple clustered close in time is more likely than not.

"viscous" when I think you mean "vicious"

Ok, I think I was visualizing this area incorrectly. I thought that Katar and co were high up on top of the landward side of this canyon which was chosen as a gap in the Wall which runs for the most part perpendicular to the sea. I was surprised that the tidal wave could threaten them there as anything more than a couple metered elevation change usually stymies the surges.

"Did this chaos fell under him?" Leaving aside the grammar issue, why does Katar feel responsibility for what he thinks is a natural earthquake? Is it some idea bout an Avatar's duty?

The paragraph where the Bold Knight shows up is confusing on the first readthrough. The "a back of an arm" was puzzling to know what you were saying.

On occasion you get too caught up in the poetry of your writing at the expense of clarity. I know that I do the same sometimes. In your desire to convey the power of the wave I think you fall into that trap here.

I liked the touch of Katar idealizing his image of the Fire Nation. It's a good contrast from ATLA.

"Shit and a shoe situation" ? I'm not familiar with that expression. I can make some guesses but not how they'd precisely apply here.

The carton of cigarettes also threw me off as an anachronism. Sorry I'm so picky about those, it's a bugaboo of mine.

All right, this chapter is a very good set-piece action sequence and throws in a few tasty morsels of hints at other things going on in the plot. Nice and very cinematic. What follows are just a few nitpicks on things that could be the tinyest bit more clear.

You do a good job describing the atmosphere in the beginning but I think the first paragraph would benefit from some front-loaded visual description of the "passage" they are in. We didn't see the characters approach this and we're suddenly inside...something. How wide is this? Is this an intentional gap or somewhere the wall fell? Is there a roof? I imagined it not having one but then later on you describe it as a tunnel.

"ambery path" I don't know what this means.

When you describe "sides" of this path, do you mean the devision is longways or shortways? Left and right walls or half of the whole path distance?

The visual emotion evoked in this segment is good, even if I am still a little disoriented.

Koursa doesn't see waterbending where she doesn't expect it. (I see later that it isn't waterbending.)

The way you describe the scar makes it sound like Kai has it.

Why is Han fighting? They're in the middle of a military checkpoint, aren't they? And the plan was always to just work the system to get across, by hook or by crook? Maybe he's just mad about the cart.

"...fingers buried in (his) hair." Without that extra word it sounds to me like the general has a very weird collection on his desk.

All that weapon stuff is nicely ominous. You've got my interest.

Ah, the earth kingdom militaries have a bit more gender inclusiveness (Major Ruki) in this time than they did in ATLA. That's interesting.

Wait, is the Bold Kight the same bald man who attacked them back in the festival village? I didn't think so until the scene returned to Koursa but her dialogue suggests that. I had thought they were fighting soldiers.

"He didn't even hit there but the pains for the kid joined in an agonizing way. Back to her." I would suggest rewriting this bit. I'm not sure what you're saying.

"What the hay" This is not an actually important issue but I think it would be "hey" instead for this minced oath although you could say that it is instead a censorship by word substitution. I'd need an entomologist to weigh in on this.

Ok, the knight is very good at that underground technique. The only time we saw that in the series was the one EarthRumble fighter and even he left a trail of disturbed surface for most of his movement. Also, it sounds like Kai got killed by that kick.

I'd think the Earth military would have a more reusable lock system for their "flush" than throwing rocks to break the levy. Something like the gate at the northern water tribe but with stone.

Careful with "croaked" as it also means "to die". In perilous situations that can be ambiguous.

You've got a nice bit of contrast between Kai screaming "SHIT" and in the next paragraph the very poetic "How it crashed, how it trembled..." It produces a seasoning bit of humor in a dramatic situation.

I think the scene where Katar imagines the flood gate being used on an army is unnecessary. You've already impressed us with the destructive capabilities, introducing an hypothetical just dampens the impact.

I'm not sure what you are conveying with the last paragraph. Is it just shock over almost dying? Some embarrassment about his performance, in not activating his Avatar powers? It just feels like you JUST barely missed hitting the target on the message.

I like the word "loominess", very amusing.

Maybe this chapter would benefit from a bit of orientation in space or time in the first paragraph? The last chapter ended with them still in the Avatar temple so it is a little confusing, a bit of a sharp cut.

I like the description of the ancient impossible wall with the trees growing out.

This first bit of dialogue is well done. Nicely paced and energetic.

Careful with desert/dessert

"...punched open the gap between floors." The way this is written makes it difficult to tell it is talking about a false bottom on the cart. My first guess was that it was referring to the wall from the previous sentences and I was trying to remember if Han was an earthbender and I had just forgotten.

How would Han's contact know to come meet them there at that exact time? Han can't exactly call ahead and I don't remember anything about him writing a letter to arrange this.

Apologies if this review seems a bit over-combative. Sometimes it is hard to get into the review headspace. This chapter is mostly just showing us some cool images and moving characters around but those chapters are necessary too. And you do a good job with it.

As a personal preference I would make the first sentence, "Koursa was trapped..." the version with the contracted "is" just sounds odd to me.

What does "I am aware." link to? In the last chapter the last bit of diologue was "Why do you even want to know?" If that is a mantra or some similar thing then perhaps you should clarify.

Koursa's reaction here seems a little odd considering this is the second time she had seen him go all glowy eyes, is it not? The first time being when they first dropped down in the cave? It just sounds here like she had never seen this before. Perhaps I am missreading.

"After a few brushes..." I'm not sure what action I'm supposed to be visualizing here.

"Where's light..." You use this twice and I believe it should be "Where there's light..." As it is "Where is light, there is ..." does not read right.

Also, how on earth did Kai get so far away? A sinkhole collapsed beneath them (odd to begin with) and now they have been walking for what feels like at least ten minutes if not far longer. Physically, how does that work, even if Badgermoles made the tunnels?

There are a few sentences where you drop the articles at the beginning. For example, "(The) Ground pulsated..." or "(The) Cave tremble..." or "(The) Wall began actively cracking,..."

What exactly was Wan's plan? Here you say it was an "attempt to trick them" but how does that match into Wan hanging upside down? I'd think they were trying to rob people but if that were the case they would have robbed Han already.

I am not used to hearing "kilos" refer to kilometers, only to kilograms but that may be regional differences.

You keep mentioning the Wall without clarifying what it is and I am starting to get the slightest bit annoyed at all the vague mysterious tidbits thrown out. Han's speech in this paragraph is really choppy and disjointed.

"Something he couldn't figure out and won't, until it's too late." I'm not sure what you are saying here. Are you literally informing us of something yet to come in the story? I am not entirely fond of predestined failure from the omniscient narrator.

I like the sinkhole oasis. It reminds me of those giant limestone caves in Indonesia.

I've only heard "dust bowl" refer to the economic/ecological disaster in the united states in...1930's? 1910's? Sometime around then. Or a few other similar occasions when over intensive agriculture caused the topsoil to be blown away into massive dust storms. If I'm right, then "dust cloud" or "dust bloom" or "explosion/burst of dust" might be better here.

"Kai clung to Katar, like a girl would to his older brother..." Setting aside the mixed gender terms here (girl/his), I'm not sure why you use "girl" here. Could be a typo. I suggest replacing girl with "child"?

There are quite a few of/off errors here. Be very careful with those as spellcheck does not catch them.

I like the atmosphere in the temple. You also seem to answer the question of the missing water avatar, just a simple case of childhood mortality. I'm perfectly willing to accept this answer though I will still keep an eye open for there to be something more about all that. It seems a little curious that there is no one in the temple despite it being regularly updated but there could easily be an in-universe explanation.

This background of Katar's life is interesting. The tense lunch is a nice scene and the chase through the house was just hectic enough without becoming too confusing. You have kept my interest on how Katar seems to have taken the Avatar's soul and powers into him and you are good once you get into the dramatic moments. On to specific comments:

You might want to spare a sentence to describe the visuals of the first space they are walking through. It is just some natural tunnel through the earth?

This is a other place where I think you need some sort of visual break between the present day and the flashback.

I like your pulling on real world folk-mythology to mention "skunk-ape". Such a creature could easily actually exist in the Avatarworld.

"Descent sized room..." First word is typo of Decent.

"Where's a dining hall..." Should be "Where there's a dining..."

"deployed his landing gear" I get what you are saying, it just feels a little anachronistic. But that's just a personal bugaboo of mine.

The most dramatic description of threatening to sit on someone, but it fit.

It might help if somewhere you clarified the relative ages of pastKatar and the "boy". Calling him boy in my mind makes him seem younger than Katar, especially since Katar was introduced to us as a grown man.

"T-shirt" also seems anachronistic, to me at least. Perhaps a different description?

The "Was he retarded?" line struck me as unnecessarily offensive and out of nowhere. It took me out of the moment.

Are "books on anatomy" a euphemism? If not, I think that warrants a very quick explanation.

You never explained or hinted that the necklace was not stored at the house so it was very odd when Katar just grabbed the key and then turned to leave without retrieving the necklace I had assumed was in his room. I flicked back to read if I had somehow missed the explanation as to how Rohan suddenly knew it was not there.

The transition to "I thought you wanted this, Koursa." is confusingly jarring. This whole conversation started with Katar under duress after he endangered Koursa's life. Now he seelms to be talking like Koursa leveraged some principle of friendship to incite this information sharing. I get that insulting the old master (Avatar?) set Katar off, but it feels like there is something missing between the previous emotional states and the ones you end up on.

Good chapter. The imagery in the north and the cave are very well done and you experiment with flashbacks mostly successfully. The dialogue works as well and you are treading the line of "mysterious" in a satisfying way. Now, on to more specific notes.

I get what you are going for with the first sudden flashback but I'm just letting you know I flicked back to the previous chapter and then started scanning ahead in this chapter because I was fairly certain at first that you had accidentally uploaded a chapter out of order. The way you say "back in the north" it makes it sound like the current location has snow just as the north has snow rather than saying this new part of the narrative takes place back in time at the north pole.

The Avatarworld term "bending" causes vocab problems here. It could just be me but when you say "She bent under the ice" I picture her physically bending over. I'd personally only use "bent" if you immidiatly follow it by the object being bent ie, "bent the ice" "bent a fireball" ect.

Very cool location under the ice at low tide (pun unintended but appreciated). I feel like that has to happen some places in the real world but would be absurdly dangerous if you are not a waterbender. The sea does not freeze where I live.

"There was plenty of there here" was/were. This section has a few areas where you drift between present and past tense; be careful.

"He began pulling the ice..." I don't know what this means. Also you switch between Koursa's perspective and the man's within one paragraph.

Maybe you could include some sort of visual break between the flashback and the return to present?

Ok, here we have confirmation Katar is the Avatar, and yet he is accused of killing the Avatar. Now, I have a bias against stories which attack the core rules of the setting but I also recognize that we could be working with inaccurate information. Could the man Katar "killed" have been a false Avatar? We know there was some funny business with skipping the water Avatar.

"Avatar's voice..." I think this needs a "The" or "An" at it's beginning since "Avatar" is a title or description rather than a proper noun.

Be careful with "he" as it refers to the last person named or described. You have a sentence where "he said to Katar" and the "he" maps either to the previously mentioned "Katar" or "the world" if it is personified. And again, "He bent down to Katar..."

I like the conversation between Katar and Koursa.

"...only but leaves..."

Not much to say about this quick little interlude. The sudden earthquake/sinkhole at the end of the chapter seemed very abrupt and out of place so soon after starting to introduce a new character but that could just be an artifact of where the chapter cut occurred. The patterns of each character are starting to get more familiar and well defined. The imagery is good if you might go the tiniest bit overboard on describing the sounds of the cart.

The grammar of the first sentence seems off from normal english structure; sounding like how Yoda would say it. Perhaps something along the lines of "The calm of the forest was interrupted by chaos rolling in."
The second sentence is a fragment. It sets up and describes a subject (the ruckus) but has no operative verb. If you wanted you could combine it with the first sentence by using a semicolon.
Towards the end of the first paragraph you slip into the present tense. Be careful about mixing tenses. Also the last sentence has an its/it's mixup.

Semicolon after "mourning".

"I'm keeping of the main roads" Typo from "off"

I'm not sure what "tarnished hair" would be.