Undernuke is an AU where every one is evil. Including Chara, Asriel, Ect.
When Asriel went to the surface with Chara and was attacked. He fought back and 14 of the Humans got scared and committed suicide he Absorbed 7 and gave 7 to Chara (Who is currently Dead). Chara was brought back to life and they took control of the surface. They made the Humans be their slaves for 5 years straight, until they went crazy and started to go into a worldwide Civil War called Surface Spread War (SSW). THey fought until the whole world was filled with the radiation of the Nuclear Bombs (Nukes). Only 77 were left alive and were complete Savages. Eventually a Human fell and he had a piece of Asriel's dust and he gave it to Alphys. Then he told the king the story of what happened on the surface. Then one day as the kid was just walking and he tripped and fell into the core (He then fused with Gaster and became Haster the Humanoid Skeleton). Then the King was so mad at his Son for turning evil he said that every single monster will be executed it break the barrier. He did this because his Son hated violence and he wanted him to pay so he wanted everyone dead. He killed about 160 of them (All of the ones that don't encounter in Undertale).
Frisk is a Human savage who craves for the flesh of monsters.
Frisk is a Human savage who craves for the flesh of monsters. But he starts to learn that some monsters flesh is not very good and only eats some alive.
Frisk is the only sane being left in the world. He tries to change people's minds about evil and fails, but at the end he shows them a Undertale timeline and they change.
Flowey is the same as Undertale except he does not explain how to move and try to trick you. He was created by Alphys.
Toriel is a goat lady that wants you dead because she wants to finally get back to the surface where she was born. And she is the one that stops Flowey from killing you because she wants to kill you herself.
Sans is like Horrorfell+Swapfell (Not the purple Sans version). Because he is pure evil and he hates his brother. He first meets you at the entrance to the ruins and shoots a Haster Blaster at you. After you run away he encounters you 17 times in total throughout the Underground the last one in the Decapitation Hall (Judgement Hall in Undertale). His fight is about 17 times harder than the Undertale version.
Papyrus is missing his left Rib Cage and he has a hole in his skull bigger than Horrortale Sans's. This has been done by Sans. He was the last one left that believed in Mercy, but the his brother attacked him and he lost Hope. Papyrus tries to poison you with Spaghetti before attacking you.
Undyne is a Killer who is always one the move getting ready to kill Humans AND Monsters! She is way stronger than the normal Undyne and Undyne the Undying does not exist. Because before she dies she says,"They're less people without me.... and that's all I wanted.