gaster is essentially the god of a cult that consists of the g. followers where he wishes to take a human soul tied with "the fallen one" (chara) so he may interact with them and erase the world so he may build a new one "devoid of any suffering - a perfect world" (almost word for word what he says in the phase 3 intro for og ulb: "A world where no one has to die. A perfect world...without you.") and your soul is connected with chara so he brainwashes sans and gets him to join the cult so he may take the human's soul