Nightmare had kidnapped Ink, Blue, and Frisk, three of the heros from the Star Sanses.

nightmare was a sans that was a melting black substance with tentacles. He was a part of the Bad Sanses, rivals of the Star sanses. The Star sanses consists of Dream, Ink, Blue, and frisk.

Dream was Nightmare's brother..

nightmare used to be kind, but was bullied a lot. The only person who cared for him was Dream. Until one day, nightmare ate a black apple, causing him to look and act himself now