An undertale AU that crosses over with Warhammer 40K.
History
Long ago, in the now Imperial world of Ebott there ruled an ancient race of xenos called monsters.
This race stood out for being an entire race of psykers who were immune to the taint of chaos thanks to being able to isolate themselves from the nefarious effects of the ruinous powers, though many of them worship the chaos gods, they are spared from mutations or insanity thanks to this mysterious effect.
They were ruled by the Dreemur royal family who took up worship to Tzeentch, because they thought the god of Hope would give them a shining future.
One day, humanity came to colonize the planet.
After a terrible war, the humans were victorious, but for reasons lost to time, instead of killing all the monsters like they had done to countless other xeno races in the galaxy, they gathered a large amount of psykers and sealed them away with a magic barrier.
Characters
Toriel: The Queen of the monsters, Toriel is basically the same as her main counterpart, she worships Tzeentch to whom she prays for the safety of all the human children who fall into the underground. Deeply hurt by her son's death, she none the less doesn't want to give up hope, hoping that one day, things will change for the better
Asgore: Again, he doesn't change much from the game. A king and a family man, he once looked up to Tzeentch for guidance but after his son's death, he jumped ship out to Nurgle.
His goal is to collect seven human souls in order to proceed with plan of the game.
Flowey/Asriel: Same as in the game, worships Tzeentch, even after losing his soul in hope he will agree to give him a new one, he plans on using warp energy to replace his lost soul after obtaining godhood, even should he somehow lose the SOULs.
Chaos gods: We all know them, but they have recently gotten a new member. They're a mix of canon and the "If the Emperor had a text to speech device" selfs. Khorne is in particular trying to destroy monsterkind as punishment for Asgore's cowardly actions via corrupting the fallen humans but so far hadn't have much success, in the genocide route he succeeds thanks to Frisk and the new god. Whatever path Frisk takes has long since been planned out by Tzeentch just like orchestrated the fall and apotheosis of their newest member. Nurgle barely finds it worth his notice but wants to help Asgore and Slaanesh simply wants to toy with them. And the new member is none other than…
Chara: The newly ascended Chaos god, of determination and stats, originally just a random human child, they fled to the mountain to escape his despised humanity. After their plan failed they was ascended by Tzeentch as part of his plans to prepare for Frisk arrival. When Frisk falls their determination awakens Chara's essence at their grave and it assist them in their quest to empty the underground, whichever path they choose to take to do so and at the end, they will welcome the child and transform them into a daemon prince, their very first. Besides being the embodiment of stats they also represent both the good side of determination (not stopping from anything to do good despite of obstacles) and the bad side (following your objectives no matter what, no matter who or how many suffer in the process). Their holy numbers, yes plural, are 1 and 7, and their runes are the heart shape of a SOUL with some decorations that would remind one of a kingdom Hearts symbol. In life they went by the nickname "The daemon that comes when people call its name".
In life they took up worship to Tzeentch because of the Dreemurs's influence, but he didn't give them any… "Alterations" because he needed them intact for his plans.
The Anomaly: An extradimensional Eldritch Abomination that influences their puppet Frisk to get the route/s they want. It doesn't come from the Warp but from none other than real life and is none other than the author, in this case ME, but it can be whoever takes this challenge.
They have time-space and reality warper powers.
Frisk: The eight child to fall into the underground. They are the catalyst to everyone's fate, the avatar of the anomaly and the pawn of the five chaos gods. They're a sweet, determined kid, at least in the pacifist route, they are constantly whispered by Khorne to go on a genocide route and are at risk of being possessed by the Anomaly or by Chara's essence, or both, when the other gods don't start whispering at them too. As their wings grow longer and stronger, they have to finish their quest before it's too late.
Undyne: Like the game. She is a Khorne worshiper, blissfully unaware that he wants a genocide run, has feelings for Alphys but refuses to show it.
Alphys: Worships Tzeentch, tis of course causes some fiction with Undyne, which is bad for Alphys since she has a crush on her, Frisk can help the 2 get together.
Among other things she experiments with the warp, usually to extract determination and other SOUL substances from it.
Mettaton: In this universe, he's a daemon bound to a robotic body instead of a ghost, but unlike his brethren he doesn't care about spreading chaos and destruction and in fact wants to be a star, he belonged to Slaanesh.
Papyrus: No changes, doesn't worship anyone.
Sans: Sans has much different changes than Papyrus. Someone driven completely to nihilism by Flowey's continuous RESETs, he has surrendered himself to Nurgle just so he can make it through the day.
Despite his great abilities he's managed to slip down the radar by being THAT lazy, but should something get him of his ass, the cause will regret it.
He is one of the very few to know about the Anomaly but he just doesn't know what to do about it.
He is also investigating and vising other AUs, he's enemies with Error, Murder and killer Sans among other evil versions of himself. He's also fascinated with reaper Sans (Being a Nurgle worshiper, he's amazed that there's a timeline where he's literally Death) and demitale Sans.
Gaster: Father of Sans and Papyrus little is known about him other than that he was the royal scientist who fell into the core and was spread through time and space. Tzeentch has the suspicion that he can be brought back using the veil of eternity due to being spread through time but so far he has decided not to do anything about it, planning his return at a later date, or maybe not.
Muffet: Worships Slaanesh, is of course, on a very antagonistic relationship with Undyne but neither has managed to kill the other.
Secondary characters
The black cat: The alternate version of the annoying dog, it's much feared in the Underground because of many, sometimes lethal accidents when it's nearby, or it can also mean that if you see it something bad will happen. It follows Frisk all over their adventure.
Monster kid: Doesn't really worship any particular god, he tries to befriend Frisk through their adventure, how well it goes depends.
The Emperor: He's a mix between his cannon self and the "If the Emperor had a text to speech device self" he finds out about the Anomaly and decides to investigate, how he reacts depends on other's behavior as much as his own personality.
Little kitten: The Emperor's loyal servant and captain general of the custodies, serves the emperor in any way he can, is extremely skilled in paradox-billiards-vostroyan-roulette-fourth dimensional-hypercube-chess-strip poker, having defeated such powerful Eldritch Abomination opponents as f*cking Tzeetch and the Emperor Himself.
Others…