This is a quick presentation of the game Undertale for those who don't know it but want to read AfterHell. All the important points are here!
If you know this universe, you can go directly to the first chapter.
What I give here is not necessarily true in the game: I took possession of the Undertale world and modified it according to my needs.
There's an illustrated version on my Archive of our own account (Naarci).
Okay, here we go!
The Undertale World
There are two races: monsters and humans. The main difference between the two of them is that monsters are made of magic, so they have powers related to magic. When a monster dies, they turn to dust.
Monsters do not look alike: their appearance can be similar to a dog, a cat, a fish, a dinosaur, etc.
The two races have souls that can come out of their body and be touched. A broken soul leads to the death of its owner.
A monster soul is a white heart with its tip pointing upwards; a human soul's tip points downwards and can be of different colors.
For the story: there was a war between the monsters and the humans. Humans won and sealed the monsters under the ground, in the Underground, behind a magic barrier which is impassable for monsters. A hundred years later, Frisk, a human child, fell into the Underground and freed the monsters.
That's it for the big lines… Now, the characters.
Frisk
She's a child about 10 years old. Her soul is filled with determination, and that allows her to travel in time. It is called a reset when she goes back in the past.
She's a great pacifist and, while the monsters of the Underground wanted to capture her, she sympathizes with each one. She frees them from their underground prison.
Her adoptive mother is Toriel.
The skeletons
Sans
He's a skeleton, known for being a big joker (his puns are lame) and a great lazybones (he sleeps anywhere at anytime). He is about 25 years old. He likes to drink ketchup (that's important u.u). He tends to hide his problems; it's his way of protecting others. In the video game, he only has one point of health (hp). One hit would be enough to kill him. He is short and smiles all the time; being a skeleton, it's his only possible expression (in other words, his smile is engraved on his face).
His magic is immense, to the point that he outperforms all the monsters (at least, if he would stop lazing around): he can teleport himself, levitate objects and living beings, create bones that can pierce and summon Gaster Blasters. When he uses magic, his left pupil grows bigger and becomes blue while his right vanishes.
The Gaster Blasters are skulls that shoot laser beams (well yes u.u).
He can make his pupils disappear from his sockets (which gives him a creepy look)
Papyrus
He's Sans' little brother (despite the size difference). He is about 15 years old. He's a naive and very optimistic skeleton, convinced that everyone can be a good person (unlike Sans, who has a quite depressed side). He absolutely wanted to enter the Royal Guard (whose mission is to capture humans; Papyrus, on the other hand, wants to become popular) but it has always been refused to him, the captain was convinced that he would rather befriend the human than capturing them (besides, that's indeed what's happened). When the monsters leave the Underground, he gains accesses to the guard status but their mission has slightly changed: they have to water the flowers.
The two brothers are inseparable. They live together and it can be said that they can't live without each other. Let's say they are the reason for living of each other.
Gaster
He's the royal scientist. We don't know much about him.
King and queen
Asgore
He is a monster entirely devoted to his people, who carries the weight of its hope. He's Gaster's friend.
Toriel
She left Asgore after a certain event (I'll pass the details, I don't talk about it in the fanfiction). She's the one who greets Frisk in the entry of the Underground and protects her from malicious monsters. When they leave the Underground, she adopts her.
Best friends
Grillby
He's a fire monster. He has his own bar. He's very discreet. He's Sans' best friend.
Undyne
She's the head of the Royal Guard and Papyrus' best friend. She's the second most powerful, just after Sans.
In the Underground, she gave lessons to Papyrus so that he could enter the Guard. Cooking lessons (yep, she undoubtedly has never planned to let him become a guard).
I think you have now all the necessary information to start reading AfterHell. If you have questions, don't hesitate!