After the monsters are set free, they aren't too keen on befriending humanity. They won't show themselves yet. They will get revenge on the war the humans had waged years ago. Now they attack in secret, getting patrols and assassins prepared at nightfall. One of the pairs of assassins is Gaster!Sans and his brother. When they find a girl who is pretty, kind, and peaceful, maybe they will have a change of heart after all. (Gaster!Sans X Reader)