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For other versions of Underfell, see Underfell (disambiguation).

"The power to heal every wound, the power to live through another day, the power to feel better after a lot of pain. I call this power HOPE."
– Dr. Gaster, explaining HOPE to Sans

"Noooooooooooooo!!! I... I don't wanna change! I don't wanna! Please! "
– Sans, after entering the second stage of the LOVE infection

Beastly Underfell is an AU similar to Underfell, but different in that their evil personalities are brought about by a virus known as the LOVE virus.

The LOVE virus is a virus that can be found by looking at a monster's SOUL[2]. If there appears to be a red slime like substance on the SOUL, that means they have been infected by the virus.

Those who are infected by the virus are known as the "Fallen Ones." The virus is stronger the more EXP you have.

If you attempt to remove the effects of the virus, it will manipulate your magic to undo your attempts.[3] The only cure for the virus is seemingly HOPE, a green substance that seems to be similar to DETERMINATION.

The virus has 5 stages:

1. Drastic change of their appearance and their magic's color, such as red eyes, red magic, and sharp fangs and claws.

2. Drastic change of their personality into something truly mean spirited and cruel.

3. Loss of their sanity for relatively short amounts of time.

4. Degradation of their sanity until they are a ruthless killer who can't differentiate between friend and foe anymore.

5. The LOVE virus crushes the SOUL of the Fallen One, and they die.

However, that is only how Dr. Gaster explains it.[4] The truth[5] is that the virus takes control of the monsters body, and forces them to watch as it kills the people they love with their own hands. And when the virus prepares to switch into the last and deadly stage of the infection, it grants the infected monster a short time span where they can act and think freely.


For most character information, see Underfell and it's character's pages.


Sans was infected as a small child while he collected medicine for Papyrus. Sans is a huge fan of custard and is the best example for the "sheep in wolf's clothing" trope.[6] He is extremely protective of his brother and can't tell him the truth about the Fallen Ones because he's too scared of losing him. Current LOVE infection stage: Four


His infection started when he was still a small child after a Fallen One attacked him and damaged his right eye-socket. He loves to create cute looking edible(!) meals. Thanks to Sans' lies ten years ago he believes that the Fallen Ones are just a huge act in preparation for his tenth birthday "tomorrow". He is waiting for "tomorrow" since then and avoids sleeping out of excitement and loyalty. Current LOVE infection stage: One


He created Sans and Papyrus as a solution to his loneliness. There's not much known about him.

Blue Belle Cat (aka "Belle" or "BBC")

Belle was one of Gaster's assistants and the foster mother of Sans, Papyrus and Alphys. She died during the "CORE accident".


Alphys is searching for a serum for the LOVE virus. She spent most of her childhood under Gaster's and Belle's care. Current LOVE infection stage: Three


Mettaton is Alphys' friend and assistant. Current LOVE infection stage: Two


His current whereabouts are unknown. Current LOVE infection stage: None


He turned into the very first Fallen One after absorbing Chara's SOUL.