Nightmarish

Nightmarish is a Boss Modifier found on Champions, Bosses, and their minions in Diablo III. When a boss with this ability hits your hero or pet, there is a percent chance that that target will be inflicted with the Horrify effect and will turn and run away from the inflicting monster.

There is no added damage or any other debuff from the attack; the danger comes from a loss of control of your character, and the possibility that they will run into something very dangerous; a cluster of enemies, or another damaging boss effect, such as Molten, Arcane Enchanted, Plagued, etc.

A Monk running scared.

The Patch 1.0.4 nerfed this ability slightly, providing frightened targets with 6 seconds of immunity to being frightened again after each incidence. This prevents characters, pets, or minions from being feared and re-feared in rapid succession.[1]

Nightmarish

• Monster Level Minimum: 2
• Available to: Champions, Bosses, and Boss minions.
• Damage Type: N/A.
• Additional Resistances: None.
• Can Not Spawn With: Vortex or Knockback.

Strategy

The only way to avoid the Nightmarish effect is not to get hit by bosses with that property. This can be tricky.

The modifier is most often inflicted by ranged attackers, since their projectiles are harder to avoid than more damaging melee strikes. Ranged Nightmarish is also less dangerous, since the enemy is usually at a distance, and your running direction will take you away from the enemy. The real danger of Nightmarish is when enemies are nearby when it hits, rendering your character helpless -- unable to attack or use any skills -- for the duration. This can be deadly against hard hitting melee attackers, especially when dangerous AoE boss effects are in the vicinity.

The most annoying ranged attacking Nightmarish boss is the Sand Wasp, since their streams of tiny bugs are hard to avoid. Other ranged attackers such as Skeletal Archers are just annoying, as they can keep Horrifying your character to run away while you desperately try to charge in to finish them off at close range.

Media

The Nightmarish effect is marked by a small screaming skull over the target's head, exactly as inflicted by the Witch Doctor's Horrify skill. There is no visual indication on the boss that they have the ability, other than in their tooltip hover text.

• 

  Skeleton boss pack.

• 

  Barbarian runs.