Nightmare

Nightmare was the former partner of Darkness. He was a ruthless killer, and had the ability to transform. He was always alongside darkness until his death by the hands of Rowan.

Partnering with Darkness
It was exactly unknown how darkness got with nightmare, but he was alongside darkness in the first attack she launched against the heroes. He demonstrated the ability to transform into a hawk. He was fended off along with darkness. He didn't appear again until later.

Death
Not as long they dont find out I was talking to you, since I'm one of the top secret species.............. Think of this next time you go and kill them! Darkness, prior to Nightmare's death.

Rowan finds Darkness, and Nightmare. (sealed in the shadows) Darkness has no intention of harming Rowan now, and explains that why nightmare is sealed, is because he almost killed one of his own kind. Rowan convinces Nightmare to be released, but he is killed upon exiting by Rowan.