Boss

The following are classed as boss monsters: Blood Raven Griswold Radament The Summoner Izual Putrid Defiler Nihlathak Uber Izual Prime Evil

• Always spawn in the same location.
• Only Griswold, Radament and Nihlathak receive randomly selected Unique bonuses in Nightmare and Hell (Putrid Defilers are treated as normal Unique monsters).
• Slow Missiles does not apply.
• -% Defense from Cloak of Shadows is not applied.
• Ignores Target Defense only applies to minions.
• -% Target Defense is only 50% effective, but still 100% effective against minions.
• Freeze length chills instead; only minions can be frozen.
• Stun length does not apply.
• Effects altering AI cannot be applied directly, but can affect minions (although Putrid Defiler minions are also classed as bosses).
• Life lost to Crushing Blow is halved (so 1/8 of current base life when applied by a melee attack and 1/16 when applied by a ranged attack).
• Life drained by Open Wounds is halved.

Prime Evil

• Holy Freeze does not slow or apply its radial cold damage.
• Hirelings only apply 50% damage in Normal, 35% damage in Nightmare and 25% damage in Hell.
• Apply 200% damage to hirelings and 400% damage to pets.

Act boss

• Andariel
• Duriel
• Mephisto
• Diablo
• Baal

World Event

• Diablo clone

Pandemonium Event

• Lilith
• Uber Duriel
• Uber Mephisto
• Uber Diablo
• Uber Baal

• Prevent Monster Heal not applied (this also applies to uber Izual). [1]

References

• Myrdinn, Ubers are immune to PMH (Technical Discussion Archives)