Gaster is an enigmatic figure in Undertale. His existence is only hinted at through scattered references throughout the game, and no one knows who he is or what he did. Some believe that Gaster created the universe itself, while others think that he was once a powerful scientist who disappeared mysteriously. Gaster is immortal with infinite HP, but he has one weakness. If you shatter his skull completely. He loses most of his powers. With a shattered skull he can no long use: Code Manipulation, Coding Error, Creation Modeling, and immortality.