Okay, since there is no official au thing from the author of the 'Fell universe, the thing is to make your own headcanon about it. This is what this chapter that's not a chapter is for; y'all can skip it if you want, but if anybody has questions it's probably going to be answered in here. If not, PM me and I will answer. I post this at the beginning of every Underfell!Au story I write even if some characters aren't in it.

Storyline

The way the Fallen Human gets to the Underground is the same way in UndertaleClassic: the human falls into Mt. Ebbot and proceeds on their journey. In Pacifist, the route is basically the same: you go through, you save everyone. In No Mercy, you kill everyone and it is exactly as expected, but Mettaton's fight is the only truly heartwrenching one. Neutral is, well... Neutral.

Characters

(In order of appearance)

Chara: Swapped roles with Frisk basically, and, in my version, they are both a she.

Flowey: The Flower who has suddenly turned nice and helps/trains you in the Underground. You also help Flowey by showing him that not everyone is evil.

Toriel: Slightly insane, mistakes you for Asriel/Frisk sometimes and makes you either Blood Pie or Cinnamon Butterscotch Pie with instead of cinnamon, it's monster dust.

Napstablook: Also called 'Madstablook'. Always angry and insecure of himself. Chara helps him realize that he is amazing just the way he is and loves his music.

Sans: Speaks with a Brooklyn accent. Wears a collar on his neck because of Papyrus, who literally abuses him in just about every form, but still loves his brother too much. Much more than he should, because of it wasn't for Pap then Sans would be dead. Loves mustard and Grillby's, just wants to be loved in a world full of hate. Has very bad cases of anxiety and depression because of his brother.

Papyrus: Head of the Royal Guard, moved the base to Snowdin and hates Grillby's. Demands that Sans calls him 'Boss' and has revoked the privilege to call 'brother'. However, with as much abuse that he inflicts on Sans, he still loves him (which is why he does it) and would do anything for him. Still a precious cinnamon roll because he tries to be angry at Chara but cannot because she is too adorable for this world.

Undyne: A fish

Alphys: A weirdass mad scientist who likes to experiment on children, but at heart, is still a beautiful dinosaur who is in love with fish chick. Only this way because of her childhood- parents neglected her and other children always made fun of her. Is the same way to MTT as Papyrus is to Sans, just less abusive and more yelling at him, telling him how useless he is as a robot who will never get a good rating. Later you meet her in True Lab and she tells you that Mettaton EX is merely a ghost inside the robot body, but can't get out because of the magic she secretly wired into the suit in order to keep Mettaton there forever as a personal sort of slave.

Mettaton: Originally shows up in box from but Alphys forces him to change to EX form because it hurts him more; tries to help you get away from Alphys while she's trying to kill you and gets caught just when you're almost away from Alphys. Alphys eventually chooses Mettaton over you and drags him off to go punish him- and what that means, Chara doesn't want to know. Mettaton smiles sadly at you as he is dragged off; you don't see him again until you get to True Lab, which happens after your Asgore fight and go for Pacifist ending. There, he's strapped into a bed, still in EX form, groaning in pain, blood everywhere. Alphys gives you the explanation of him and you are forced to leave; however, you come back at the end and rescue him from the lab and the suit and give him a regular body that he can show off in the human world.

Asgore: King Fluffybuns. Basically Toriel Classic, with a few tweaks such as letting you go through the barrier without a fight. You do fight him in True Pacifist, and end up letting him take your soul along with all the others. Eventually you get a body back and live happily on the surface with him, Papyrus, Sans, Alphys (though you're still getting to trust her), Undyne, Mettaton, and Flowey (who still doesn't trust anyone for shit).