Trepidtale is an AU similar to Underfell, yet completely different. It is set before Frisk descended.
"Long ago, two races ruled over Earth: HUMANS and MONSTERS. One day, war broke out between the two races. After a long battle, the humans were victorious. They sealed the monsters underground with a magic spell."
– The opening
The king, enraged about the loss of the war, called up one of the most intelligent monsters in the Underground, a young skeleton named W. D. Gaster. Asgore hired him as the Royal Scientist, and ordered him to figure out a way to escape the Underground. Gaster was unsuccessful many times, which resulted in a beating from Asgore. One day, the young scientist finally discovered how to escape. 7 human souls. Asgore ordered any monster that discovered a human to kill them and bring their soul. The monsters have found Patience, Bravery, Integrity and Perseverance. With Kindness, are you able to either end the monsters killing innocent humans by killing them or befriend the entire Underground?
- Gaster has not shattered across time and space, and is still the Royal Scientist.
- Because Gaster is still around, Alphys is unable to create Mettaton, thus Happstablook exists instead of Mettaton.
- A battle with Sans occurs during all 3 routes, where Mettaton would fight you, and a Gaster fight is in the place of a Sans fight during the Genocide Route.
- Throughout the entire game, Gaster is your only ally.
- Despite the canonicity of Undertale, Chara and Asriel are alive.
- The Mad Dummy and the Ruins Dummy are ghosts.
- Napstablook, Ruins Ghost and Happstablook do not have a real fight.
- ACT is replaced with DEFEND.