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Underworld is (now) based around the idea of a multiverse creature corrupting the monsters of the Underground, resulting in the game’s characters becoming horrifying monsters whose only goal is to hunt down and kill Frisk.
After an unknown creature that lurks in the Multiverse corrupts the AU, Frisk goes about their usual pattern and descends onto the rough ground, breaking their arm. They then find a wilted, talking, golden flower, who assists them in tying off their broken arm. They later find a grotesque goat monster, and the story unfolds.
Frisk has remained generally the same, except that their arm is broken and covered with blood.
Flowey changed for the better, as he has somehow learned compassion and pity. He takes the form of Flowey after you beat Omega Flowey on a neutral run.
Sans is now taller and stronger. He has a huge jaw, a glowing orange and blue eye, and has a crack in his skull flowing with energy.
Papyrus now has no skull nor arms, and his head is replaced by black liquid flowing out of his spine.
Alphys has black liquid flowing out of her mouth, and the amalgamates from her true lab are stuck to her like parasites.
Undyne looks like a deep-sea angler with giant teeth, and since the only possible path is genocide, she is always Undyne the Undying.
Muffet is now a huge spider, with an enormous teapot shaped body.
Toriel is a massive, grotesque, monster. She wears the same kind of outfit as Classic Toriel does. She has a more goat-like head with four long horns on her head. She has no lower jaw, which is why her tongue is always sticking out, and she only has one arm.
Mettaton does not live inside his normal robot body. Instead, he lives as his ghost form, with a heart eye patch and a fringe. He also has teeth like the Mad Dummy has in his body.
Napstablook has three eyes, all crying black liquid.
Asgore still looks relatively the same, however, he has a massive mouth full of razor-sharp teeth, with an extra jaw for a tongue, and his hair has grown quite long. He also wears the six human souls on him.
Monster Kid has giant teeth and his horns are damaged.
- The only possible path is genocide.