Nightmare is a boss in Slender Fortress.
Nightmare's appearance is identical to Nightmare Fredbear's appearance, only the costume color is a translucent black, and the bow-tie and tophat are changed from purple to yellow.
If images featuring Nightmare were brightened, it would reveal that Nightmare's costume is actually translucent, as if his endoskeleton is cloaked in a shadow. As such, nearly all of his endoskeleton can be seen by brightening images of him. The only exception is around his mouth, which looks like an existing material was ripped away from it.
In Slender Forteess Edit
Nightmare is similar to Nightmare Fredbear, but he is black and has a golden bowtie and top hat, he can't be stunnable untill he has helpers called, Nightmare Freddy, Nightmare Bonnie, Nightmare Chica, Nightmare Foxy, Nightmare Fredbear, Plushtrap, Jack-O-Bonnie, Jack-O-Chica, Nightmare Balloon Boy, Nightmare Mangle and Nightmarionne.