Poison Enchanted (2)

General Information
Poison Enchanted is a monster bonus used only by Achmel The Cursed and Lilith, random Unique monsters can't have it. It's not shown under the name. This will imbue their attacks with the element of Poison, now dealing damage of that element. It also adds an Poison aura/invisible Poison projectile to them.

Diablo II/Lord of Destruction
Poison Enchanted will grant the Unique monster the following bonuses:

Poison Damage: x2

Chance to Hit: 66% to 100%

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

The Unique has an invisible Poison aura.

Poison Enchanted is not granted to minions, unlike other Elemental Enchant modifiers.

Diablo III
It is completely different in Diablo III. Refer to this page.