Monsters change between the difficulties, and bosses spawn with all sorts of interesting properties.

Hit points:

   Minions: x3 (Normal)
   x1.75 (Nightmare)
   x1.5 (Hell)
   Champion: x3 (Normal)
   x2.5 (Nightmare)
   x2 (Hell)
   Unique: x4 (Normal)
   x3 (Nightmare)
   x2 (Hell)

Level:

   Champion: +2
   Unique: +3

Experience:

   Champion: x3
   Unique: x5

Champion Monster Bonuses

Champions:

   Damage x2
   +33-50% Elemental Damage
   Attack Rating +75%
   Attack Rate +120%
   Velocity x2
   +20% Speed

Ghostly:

   Half Speed
   About 20% Chance for Cold Damage
   50% Physical Resistance

Fanatics:

   +100% Speed
   -70% To Attack Rating

Berserkers:

   Damage x4
   Attack Rating x4
   Hit Points x1.5

Possessed:

   Hit Points x12
   Can't be Cursed

Unique Monster Bonuses

Boss Properties are semi-randomly generated. Random bosses spawn with 1 random property on normal, 2 on Nightmare, and 3 on Hell. Superuniques generally have one or more fixed properties at all times, and many of them gain an additional random 1 on Nightmare and 2 on Hell. Act Bosses and Uber monsters do not get random properties.

Aura Enchanted

Aura Enchanted Monsters may have the following Auras:

Might (at monlvl/4), Holy Fire (at monlvl/2), Blessed Aim (at monlvl), Holy Freeze (at monlvl/2), Holy Shock (at monlvl/2), Conviction (at monlvl/4), and finally Fanaticism (at monlvl/4)

Cursed

f Unique hits, 50% of the time, an Amplify Damage Curse of (monster level / 4) is cast on attacker.

Cold Enchanted:

Cold Minimum Damage: +66% damage Cold Maximum Damage: +100% damage Cold Length: +20 Chance to Hit: x2 Cold Resist: 75% If dead, launch a Frost Nova of Monster Level.

   In Nightmare and Hell, Minions gain +33-50% Cold Damage.

Extra Fast

   Unique and Minions get a velocity increase
   Attack Rate x2 (unique only)

Fire Enchanted

   Fire Minimum Damage: +66% damage
   Fire Maximum Damage: +100% damage
   Chance to Hit: x2
   Fire Resist: 75%
   If dead, does damage of 75%-100% of max hit points, in a radius of 4 yards.

   In Nightmare and Hell, Minions gain +33-50% Fire Damage.

Lightning Enchanted

   Lightning Minimum Damage: +66% damage
   Lightning Maximum Damage: +100% damage
   Chance to Hit: x2
   Lightning Resist: 75%
   If Monster is hit, a Monster level Charged Bolt is released.

   In Nightmare and Hell, Minions gain +33-50% Lightning Damage.

Magic Resistant

   Fire Resistance: 40%
   Lightning Resistance: 40%
   Cold Resistance: 40%

Mana Burn:

   Mana Minimum Damage: +(Minimum Damage x 4)
   Mana Maximum Damage: +(Maximum Damage x 4)
   Chance to Hit: x2
   Magic Resistance: +20%

Multishot

   2 or more missiles of the same type are shot.

Spectral Hit

   Fire Resist: +20%
   Lightning Resistance: +20%
   Cold Resistance: +20%
   Chance to Hit: x2
   Pick from Fire, Lightning, Magic, Cold, and Poison and do:
   Element Minimum Hit: +66% damage
   Element Maximum Hit: +100% damage
   In Nightmare and Hell, Minions gain +33-50% random elemental damage.

Stone Skin

   Damage Resist: +50%
   Defense x2

Extra Strong

   Minimum Damage: x2.5
   Maximum Damage: x2.5
   Chance to Hit: +25%
   Minions do +75% damage.

Teleport

When (health < 33%) or (ranged monster & someone is close), teleport. Also, if (health < 33%) add some hit points to health.

This property was very difficult to deal with in early versions of Diablo II, since it let monsters regain their full hit points each time they teleported. Battles between low damage characters and high hit point teleporting monsters could go on almost indefinitely. This property was nerfed in subsequent patches and

Some monsters, such as Baal and Uber Izual, will teleport without regaining hit points.