Gaster is the royal scientist of the underground. He was coming home mush more late due to his work. One night once he got home he seemed to call for you downstairs, you had already put Papyrus and Sans to sleep. You're a (monster type of choice) adopted by Gaster while Sans was a babybone. Gaster wanted to test the prototype of what he calls 'Determination pills', no one would volunteer so he decided to try them on you. The pills would make you feel like you're burning once you take one.