[Stabbsworth]

There's three different main endings to the game. Pacifist involves the Monsters going free from the mountain, Neutral ends with the Monsters staying under the mountain due to the lack of souls, Killer involves you destroying the game universe.

There's multiple branches of Neutral endings, but the main result is the same.

Gaining any EXP during the game makes the Pacifist route impossible to complete, and a Neutral Route needs to be completed first. Additionally, you must also hang out with Papyrus and Undyne, deliver Undyne's letter, complete the date with Alphys and go through the True Lab.