The Nightmares are some of Korrigahnn's most powerful servants, who are encountered later in the game. They rely on shadows and terror to fight minifigures.
When Korrigahnn betrayed the other Great Builders, the servants of each took sides. Those who aided Korrigahnn were given the task of protecting the Great Builders trapped inside the Dreaming. There, they began to mutate, the energies of the Dreaming blackening their skin and giving them batlike wings and bladed hands, a terror to all the prisoners. With his normal soldiers being defeated on Acrea, Korrigahnn has sent some of these into the "daylight" world, as part of a trap to lure the minifigures to the Dreaming. Outside they wear they wear hoods and cloaks to protect themselves from the natural essences. When the attack on Korrigahnn's fortress begins, Korri sends most of the Nightmares to terrorize the attackers while he waits inside.
The torso and legs of the minifigure are "very very dark gray", with a slight reddish tint. There are three grey talons on each of their feet, and while one of their hands is normal, the other is curved like a blade, and can be used as such. They wear black cloaks that cover their heads and most of their torso, the only visible part of their head is glowing red eyes. They also have black wings, the tips of which glow red in the darkness, to form a frightening silhouette.
(I'll add how they attack later.)