Monsters on Nightmare Difficulty
On Nightmare Difficulty, Monster Level, hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.
Monster skill levels go up +3.
Unique Monsters gain one bonus ability.
Monsters on Hell Difficulty
On Hell Difficulty, Monster Level, hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.