So this is a small idea I've been wanting to try out, given ToG's vast selection of characters and powers and plot directions, and Undertale's various possibilities. This will be a oneshot, since I have no plans for a full series, since I think I won't be able to write up a lengthy enough plot yet, neither do I think it would be popular enough for me to work on. Anyways, hope you like it, and if you have any suggestions, feel free to leave em, I'll be happy to respond to them all!

Disclaimer:- Tower of God belongs to SIU, and Undertale to Toby Fox. I only own this crossover story. The cover image is from the Undertale AU wiki.

Floor 133

The figure of a young girl, with ruby red eyes; wavy, raven black length hair that was tied in twintails reaching till her mid back, wearing knee length black stockings, a black shirt adorned with a blood red tie and a white overcoat with red linings along its edges materialized in the 133rd floor of tower. Accompanying the girl was her guide, Evan Edrok. The most noticeable thing about this man was that he looked like a grown man whereas the girl like someone who was in her teens. Yet, they both were about the same height. He had dull silver colored short hair, a boorish look on his face, steel grey eyes and milky colored skin. He also carried with him a backpack that was clearly not meant for his size, yet he didn't show any sign of stumbling under it's weight.

The girl was eager to take on the final test of her ordeal as a Regular, in order to claim her position as a Ranker. It had taken her a mere 200 years to reach this point, a feat which many claim is worthy enough to be branded as a High Ranker. Her partner had accomplished this feat in a similar 175 years. Yuri Ha Zahard's heart pounded, anticipating the kind of monstrous challenge that awaited her. Evan absolutely refused to say anything, telling her that this was an ordeal she had to pass on her own. As the 2 looked around, they noticed that the floor itself was quite small; Just a long narrow hallway, with square tiles lining the entire floor, cream colored pillars on both sides near the sides of the room supporting the roof. The roof itself was curved like the top half of a dome. The right side was a solid wall, but the left one had humongous windows with arches, and a crest of sorts, one that she'd never seen before. The entire room was illuminated by a golden light flooding through these windows.

"That's strange…I don't remember the last floor being like this" mentioned Evan, scratching his chin. "It doesn't even look that big…nothing like any of the floors I've seen in the tower"

"What? You mean this wasn't the same test you took? Does this mean that the test changes with each person?" asked Yuri, curious. "And you're right, I wonder if this is really a floor….it's soo tiny. The halls in the Zahard palace were much bigger than this"

"I'm not entirely sure princess. I've never heard of this happening before, although it doesn't seem entirely impossible. After all, it's all in the discretion of the administrator of the floor" he replied.

"Heh, it matters not! No matter what awaits here, I'll be the one to emerge victorious!" she proclaimed, and Evan sighed. "Still, I don't see the test administrator anywhere….could that be the test? To find the test administrator?" she said to herself, moving along the corridor, looking around.

*heya!

came a voice, from a nearby pillar. In said pillar stood a most unusual person they'd seen. It was the figure of a skeleton, wearing a blue overcoat with white fur around the neck, black colored shorts with white stripes along the edge, and blue colored slip-ons. It sported a wide toothed grin, and was reading a book called 'A Jokebook on quantum mechanics', slanting along the pillar.

*the name's sans, sans the skeleton

"Are you….the test administrator?" asked Evan, uncertain.

*you could say that, although i consider myself a professional prankster. so, here to take the test i suppose? still, it's rude not to introduce yourself, don't you think?

"Ah, apologies. She's Yuri Ha Zahard, here to take this floor's test, and I'm Evan Edrok, her guide"

"That's right, bone-head! Tell me what's it about already!" shouted Yuri enthusiastically, shaking a fist in excitement.

"Princess!" said Evan in caution, worried about her brash nature and also about insulting the test administrator.

*thanks for the compliment, kid

Now it was Evan's turn to sweatdrop. It seems everyplace he went, he ended up meeting one weird person or the other.

*'preciate for droppin by kid, but if you want to go up, just take those stairs, you'll pass. its too much work, so i don't really feel like it

"Hah?!" gasped Yuri, partially surprised and partially angry. Evan was equally shocked as well. "Are you mad?! I didn't come all this way only to be treated this way! Fight me or are you scared?!"

"Princess calm down!" sighed Evan, and then turned to him. " What do you mean, you don't feel like it?!"

*exactly what i said. it really wears me down….to the bone

The duo groaned at that pun, and Evan could've swore he heard the faint sound of a instrument complimenting that joke.

*it's fine, i got permission from the administrator, so its all good. and besides,

The atmosphere became dark and cold, sending a chill down their spines. Sans's eyes were gone, replaced by darkness.

*you REALLY won't like what happens then

Meanwhile, the princess was not doing so good. She was already pissed off at the lack of his interest and carefree attitude to her, and upon that, she thought he looked like a joke to be the final boss of the tower. That chill on her spine only made her more excited to test this guy out, and make it worth overcoming the obstacle that was the final floor.

"Argh! THAT's it! I'm coming at you, whether you like it or not! Pray that I don't break too many of your bones!" she declared. The skeleton sighed.

*welp. there's always that one that doesn't want to make this easy eh? this is turning out to be a pain…

Said sans to himself. Yuri rushed in, with her trusty needle Black March, aiming a stab at his head. Sans avoided it with almost no effort, too fast for even Evan to see. The momentum of her strike carried her forward. She regained her stance, shocked at his reaction speed.

"What the….?! He avoided that so easily….me, a princess of Zahard? He's no ordinary guy after all"

"…..I can already tell by looking at the princess that she couldn't follow what just happened…but to be invisible to my eyes as well…?! That would mean he's at the level of a high ranker" thought Evan to himself, surprised.

Sans thrust his arm forward, and the black haired beauty felt her entire body being forcibly propelled backwards, sending her crashing to the wall. The impact itself was not that damaging, but she felt the wall behind her quiver, and multiple bones pierced outward. If not for her reflexes being boosted as a Zahard princess, she'd most certainly have been dead. But even so, she wasn't completely unharmed, with her right foot having been stabbed by the bone attack.

"Was that…shinsu?" questioned the silver dwarf to himself. "he appears to have telekinetic powers as well….what IS he?!"

"You…what the hell was that!?" asked Yuri, limping a bit.

*you REALLY should've advanced when i told ya so nicely.

"No freakin' way! Now that I know you're strong, I'm not leaving until I break your bones!" she challenged. Sans laughed a bit at her bone pun, even though it was unintentional. But at the same time, he sent multiple pillars of bones, all made from shinsu, hurling towards her at multiple speeds, with small openings in each pillar from either side of the room. Gripping her needle, she rushed at the nearest and slowest one, gracefully passing through the opening in the pillars, at the same time, keeping note of the ones advancing from her back.

Pressing forward, the black haired beauty jumped in the air, with her hands above her head.

"Yuri Ha shinsu skill, Rose Shower!" she cried, and a massive torrent of rose shaped shinsu rained down on where Sans stood, sending dust and debris everywhere. The attack left a sizable crater in front of her, which told her he couldn't have avoided it without damage.

Except he was nowhere to be found. She felt her body being forcibly slammed to the ground, but unlike the last time, she knew what was coming and jumped to avoid the bone spikes. Sans too, wasn't quite yet done. Yuri was thrown against one other wall, and this continued until she was practically thrown around the 4 walls of the corridor, bone shaped spikes emerging from each one of them. She received injuries to her right shoulder, her left palm, her forehead and her right thigh. When the onslaught stopped, she noticed that her injuries themselves were not that painful, about as much as a pin prick. But, overall, with each attack she took, the damage increased more and more, and so did the pain. But this pain was more 'mental' than physical. Sans stood, still wearing that grin he had from the start of their fight.

*i suppose you're feelin' it now…your sins crawling behind your back

"What…do you mean?" breathed Yuri, panting.

*a little concept called KARMA. what goes around, comes around, somethin' like that, get it? my attacks are inbued with them. to get to this point, im sure you mustve done some pretty nasty things; hurting others and even kill people. while each hit from my attacks only hurts as much as a pin prick, you also feel the increasing weight of your sins due to KARMA.

"Urgh…" groaned Yuri, in pain. "I came here, sacrificing so many things…like you said, it's true that to get till here, we had to hurt others, and even kill people…but this is precisely why….I cant give up here and now! Otherwise I wouldn't be able to face those that I have turned my back on!" she said defiantly, picking herself up from the dirt. Gripping Black march tightly, she rushed at the skeleton, delivering a quick and powerful barrage of strikes, slashes and stabs, at inhuman speeds. She gave it her all, completely abandoning her guard and focusing only on striking more faster and deadlier. Yet, this didn't faze Sans at all, who dodged them all many times faster. She felt nothing but air in all her strikes. Switching her tactic, she combined some strikes with shinsu attacks, manipulating their trajectories midway in hopes that one of them hit him. But to her shock and dismay, this didn't work either, and the skeleton continued evading her attacks.

"This is preposterous! How the hell is he so fast?! He avoided Princess's Rose Shower, which is something near impossible to evade in such a closed space as this. Then he evades all of princess's attacks which move in flexible trajectories and varying speeds and have shinsu attacks mixed in them…..how in world did I miss someone this outrageously strong?! And the nature of his abilities….could it be…that he's an irregular?" thought Evan shocked, his eyes unmovingly observing the fight. It was forbidden for a Ranker/Guide to interfere in a Regular's exam, so there was naught else he could do.

*give it up…this can go all day. Said sans, moving back. Waving his hands, he conjured up multiple skull shaped heads, with glowing blue eyes, and wide mouths. They closed in on Yuri from all over the room, one after the other. Each one opened its mouth, a beam of brilliant white light growing in intensity. Her gut feeling told her to get out of its path, and she performed a dive roll to the nearest pillar. The skull let lose a beam of powerful shinsu at an incredible speed. Remembering what sans said about his attacks having KARMA, she theorized that she would be fine even if she did get hit a bit in order to close in on Sans and deliver a blow. But that beam of shinsu was bad news, it's power atleast as much as her own shinsu blasts.

One after the other the skulls opened fire, blasting away from different incredulous angles, one she had trouble anticipating and avoiding. It was taking all her effort to simply evade them one after the other, let along give an opportunity to attack. Once the barrage of blasts ceased, she wasted no time, and scanned the hall for any sign of the pun-loving skeleton.

*over here

A bony finger poker her shoulder, making her flinch rather strongly. She turned in surprise, only to find the open mouth from one of the skulls staring at her, an orb of light at the center. The beam hit her squarely, sending her flying through a few pillars. She coughed up blood violently, struggling to pick herself up.

"What the hell is going on…..why can't I reach him with any attack?! Not to mention, his speed is ridiculous as well! I've never seen any Ranker even move that fast?! Not even Phantaminum….who the hell IS he?! An irregular?! Still, that last one really did me in…I can barely feel anything….but there must be a way…there has to be!"

*you're….very determined to continue aren't you? you look like the type that keeps going just because you 'can'…..and because you 'can', you feel that you 'have to'. take my advice kid, and give up. accept that you've hit the end of the line, and you'll feel better

"N…No way….! I c..can still keep going!" she retorted back, stubborn. Sans sighed sadly.

*guess you wont learn this the easy way

Yuri felt the familiar pressure of being slammed to the ground by an unknown force. She knew that bone spikes were going to pierce her from below, but she didn't have the strength to avoid them completely. One of them hit her elbow, sending a sharp pain throughout her arm. Next, a massive bone pillar, with openings of varying heights moved in on her, and she knew she couldn't avoid it perfectly, so she instead chose to leap straight through the middle of the path, where she got hit here and there, minimizing the damage. The pain due to KARMA was adding up slowly. Immediately after, 8 of those skull blasters closed in on her from different points, firing off beams one after the other in varying patterns. Some of them grazed her, but the last one, where 2 massively sized blasters prepared to fire off right in front of her, giving her no room to dodge. Evan watched in shock ,wondering whether Sans was really going to kill her. He had to hold himself back to his limit to prevent himself from interrupting the exam.

Yuri on the other hand, looked at the ground in bitterness, and dropped her needle, giving up. She looked like she was in the verge of tears, not from her injuries, but from her pride. The light from the blasters faded, and sans had a more softer tone.

*that..is the right answer to this test. you have learnt, and thus you are qualified to pass.

Yuri jerked her head up, in surprised and in disbelief. Evan came running to her, looking to tend to her injuries.

"W..what do you mean?" she breathed.

*while being determined to face an adversary above you is no small feat, it is harder still to convince yourself to retreat when the need arises. if you are aiming to be strong, you must learn to accept yourself when there is a wall you cannot climb. the fact that it is ok to turn back, and try again. the fact that this is your limit. and also the courage to accept the consequences of your actions and all the deeds you did to get here. if you turn a blind eye to these feelings, you are not qualified to be a ranker of the tower. you know, and you pass.

"Bu..but I haven't beat you.." she replied, still unconvinced. "If I don't, there's no meaning in me going further"

*seems like you still aren't convinced….so lemme tell you something to chill your bones. even the likes of your father, Zahard couldn't land a single blow on me, let alone injure me. although to be honest, i'm not that strong physically, and probably the weakest in that category. The pun didn't even register in their brains, the statement about King Zahard completely washing it over.

"W..wait what?! Not even the king of this tower could touch him?! I was right, he really is an Irregular, and one who's mostly at the same level of Enryu or Phantaminum….to think someone like this is the test administrator of the final floor…..seems like things changed in ways I could've never predicted…" said Evan to himself, speechless and shocked.

"N..no way…not even father could beat you…?! Impossible…..I assumed you might be as strong as Phantaminum before, but now I guess it's not all that unbelievable…" she gasped. "What's someone like you doing here?!"

*well…let's just say that being here is more fun than where i come from. nyways, looks like your navigator pal patched you up well enough, although i do feel bad about crossing my BONEdries

Evan and Yuri groaned at the pun.

"But then, what do you do to those that chose to simply go to the next floor unlike the princess? Do they fail automatically?" asked Evan.

*nah, when there's a chance where i can avoid the hassle of testing, why would i let it pass by? of course i don't stop them, as long as it means that i get more time. He chuckled. Evan and Yuri sweatdropped.

"I didn't find that HUMERUS" replied Yuri, a vein in her head, and also retorting to Sans's with a pun of his own kind.

*Now THAT tickled my funny BONE!

replied Sans, and the two engaged the other in a pun war, until it was painfully obvious that the bone head would be victorious. Once her injuries healed well enough, she took her leave, but the atmosphere between the Zahard princess and the pun-loving skeleton was much more relaxed. Although this experience may fade over the many years she would live later on as a High Ranker, it would never completely fade away. With new lessons to learn, experience to gain and her resolve stronger than ever, she awaits the day she encounters the enigmatic, joke loving, ever grinning skeleton known as Sans.

That's it folks! Now I tried my best, and like I said, I don't plan on writing a big series anytime soon, just wanted to see the kind of reception this kind of crossover gets. I hope you liked it, I tried to keep all characters as close to their original personalities as possible, and do apologize if I made mistakes here and there. Perhaps if I had time to work on a bigger plot, it might have come out better. Thank you so much for reading!