"If you get hurt, you must hurt others as well. Calamity surpasses everything." — Nightmare!Sans threatening Cross and Dream!Sans Nightmare!Sans is the secondary antagonist of Underverse Season 1, and a master for Cross and X-Event!Chara. After Cross ends up having negative feelings, Nightmare enters his lonely universe, and they both create a deal that will allow them to extract parts of other worlds to his. He doesn't start causing problems until during Underverse 0.3.