Grimnight the Soulless, Shadow of Death is a unique Enslaved Nightmare found in The Silver Spire Level 2 in Act IV of Diablo III.
In combat, he has Fast, Thunderstorm and Teleporter traits, and can summon other demons more often than normal Nightmares.
A bounty may be offered to kill Grimnight, in which case the secondary objective is to kill 100 enemies.