Spectrotale is an AU where the monsters are sadistic and evil, and even perform unethical experiments on humans.
As the name implies, it was created by Spectron (canon to..idk). He made it out of boredom. In real life, it was created by Fandom User Vincent Endethyst for the same reason.
About the AU
Most of the characters are sadistic and cruel.
The monsters originally lived underground as oppose to originally living on the surface then being driven to the underground.
Only Spectron can use magic here. Any other being who enters will have all of their magic suppressed. Natural abilities will also be suppressed. This is because a monster tried to revolt against Spectron, nearly killing him.
They monsters constantly attacked and terrorized humans for centuries.
One day, a man by the name of Alexander grew fed up with the monsters constantly killing and tormenting his people of his village, so he sealed the monsters underground with a magical barrier using his incredibly strong magic and demanded all monsters still on the surface be slaughtered at once.
Unlike the original Undertale, the monsters in Spectrotale see no reason to break the barrier, so they do not harvest human souls. However, anytime a human comes down, they usually end up either killing or torturing them.