Undertomb is an AU, where the characters of all the characters became more bloodthirsty.
Major changes have occurred to the monsters. These creatures live by feeding on the bodies of dead people. Some live underground, campfires and various lamps. Most monsters underground hideous and distorted, the smell of stale blood. However, there are sane monsters who are trying to control theirselves. King is trying to create some underground laws, but all monsters ignore it. Many of them feed on corpses, but there are those who are simply different from the original.
The sun is out, instead - Red Moon, which is constantly paints the surface red, also captured monsters face and fight with people.
Characters wear a lot more in the means of dark colours, with blood on most of them. Eyes are mutated or mutilated, and are often bright red, solid black, or a bit of both. They smell like decaying flesh and feast on dead humans. On the surface, there is no sun, just a blood red moon. Zombies and other supernatural creatures walk the earth, sometimes falling to the underground to feed the monsters trapped below
The characters have not changed either for the better or for the worse, they just tweaked a terrifying style.
Grouchy and lazy guard in Snowdin, the eldest among the two skeleton brothers. He received his nickname "Teddy" from the newly fallen human, whom he sometimes helps and protects against other monsters, but he usually just stands and watches. Sans still enjoys jokes and puns but he has a darker, more twisted sense of humor. Gaster blasters like the big cats. They have their ambitions, eyes glowing red, his mouth has mandibles.
He still aspires to be a member of the Royal Guard, but, like in the original universe, Undyne can't bare to see him get killed so she just keeps him on sentry duty. Papyrus is still rather sweet in his own strange way and is unintentionally dangerous.
Fallen Human. Equipped with blue gem on the neck, a cross and an old lamp working on kerosene.
Recluse Ruin. Buckish monster that at a meeting with Frisk, sparing her and advises not to fall Asgore. She has a scar on her face, like a satanic cross. It looks scary, but kind and caring.
Demonic, evil and soulless creature who hates mankind and monsters. Is reincarnation Asriel Dreemurr.
Demon Life-after-death, all the way to the asylum in revenge. Is inverted cross and the sword. They have black wings and flowing from the eyes of black sludge.
Prince of the underground, Son Toriel and Asgore. Like all healthy monsters and their children, his diet includes human body parts. Blind because of the hideous scar on his eye. If he was ever able to grow up, Asriel would have became more terrifying.
The most dangerous creature in the Dungeon. The face is a mask that hides a terrible and incomprehensible monster. Lazy, so do not leave their village. Everyone fears them.
The merciless head of the Royal Guard. Blinded by Asgore, but well developed all the other senses, which makes her a quiet and quick killer. She has small wings and is secretly in love with Alphys.
Royal scientist. The character was canonical. Serves Asgore out of fear and the vain hope of getting new eyes. She secretly has a crush on Undyne
King Dungeons. Merciless, mad, and always strongly hungry monster. History meets the canon, but greatly marred. Quite muscular.
Lost Cousin Mettaton. Familiar with Toriel and often staying with her.
Serial killer robot, a magician and television star. The nature identical canonical.
- Before Sour-Apple-Studios version, Undertomb was originally called "Horrortale," but when the Sour-Apple-Studios' Horrortale had become popularity, the creator decided to renamed this AU into "Undertomb."
- The name "Teddy" came around when Minty and Shorty were discussing a completely different character altogether and Minty joked that Sans should be called Teddy for the irony factor and it just stuck.