Nightmare's a skeleton monster with only a single glowing cyan eye light within his socket. Coming out of his back are six powerful black tendrils. His bones and clothes entirely black, covered and dripping of a viscous black tar like substance. He wears a t-shirt, shorts and hoodie. He's the leader of the Bad Sans/ Nightmare gang. Their purpose is to cause negativity through the multiverse to feed Nightmare and his power. They're enemies to the Star Sans who wish to protect the multiverse. 