Enslaved Nightmares are demonic enemies encountered in Act IV in Diablo III. They are sorcerers enslaved by the demonic corruption, and while they are in service of the Evils now, they still retain a portion of their sanity.
"The Enslaved Nightmares stand as testament that there will never be peace between men and demons. Sorcerers who attempt to master the dark arts and summon demons with their black spells are all damned to become skeletal nightmares bound to the very power they tried to bend to their will."— Deckard Cain(src)
Nightmares are only fought in the Silver Spire and Besieged Tower, often appearing from portals when you do not expect them. They are fragile, have no melee attack, and try not to approach the player, hurling piercing darkflame bolts from a distance (dealing Physical or Fire damage, to which you are more vulnerable to). If stationed on bridges, they may launch slower, but wider and more damaging waves of flame in rapid succession, blocking the path.
In addition, each Nightmare can open a portal every 30 seconds (but not in the first 10 seconds of combat), through which other demons can arrive at his side: Armored Destroyers, Oppressors, Shadow Vermin or even Terror Demons, so killing them quickly is advised, before they can summon more enemies. When the demons are summoned, Nightmares can shield them (or themselves) with Bone Armor-like spell which absorbs a certain amount of damage.
However, their most dangerous ability is putting one of four curses on a hero. Each of these curses will reduce a characteristic for 5 seconds, and there is hardly any method of reliably avoiding these debuffs. Only one curse can exist on a player at any time, however.
The curses are as follows:
- Curse of Rust: reduces Armor by 50%
- Curse of Mud: reduces Movement Speed by 80%
- Curse of Resistance: reduces all Resistances by 50%
- Curse of Weakness: reduces damage done by 25%
Upon death, they leave no corpse: rather than being killed, they are sucked into the dark swirling portals, presumably being drawn to the Black Abyss.