Cold Enchanted

Cold Enchanted is a monster bonus available to powerful monsters throughout the world of Sanctuary. This gives their attacks the Cold attribute, slowing heroes upon a successful hit. This bonus is deadly in that once it hits, a hero will suffer substantial penalties to both attack speed and movement speed.

Diablo II/Lord of Destruction
Cold Enchanted grants the following bonuses to the Unique Monster:

Cold damage: 66% to 100%

Cold slowing length: 20

Chance to Hit: x2

+75% to Cold Resistance, they don't gain more resist if it would cause a third immunity or increase one of two immunities.

When the creature dies, it releases a equal to its level. This nova can prove fatal at worst and hinder players at best, so it is important to keep health levels high before dealing the killing blow. Half Cold Duration and/or Cannot be Frozen affixes on your gear will keep the hindrance to a minimum as well, although this may be difficult to attain.

A great example of a monster that has caused many troublesome fights due to this bonus is the Super Unique Monster, Coldcrow. She and her minions alone can certainly be a good reason as to why a player might want to ignore the Caves in the Cold Plains on higher difficulty settings.

In addition, on and  difficulties, minions gain 33% to 50% Cold damage. Uniques also gain an immunity to cold in Hell.

Diablo III
Cold Enchanted has been changed in Diablo III:
 * It is split to Frozen and Frozen Pulse affixes, with different mechanics.
 * Champions can also spawn with this, in addition to Rare monsters.
 * Basic attacks no longer cause additional Cold damage.