Omen3
A lightweight, flexible, multi-target threat meter.
Mists of Pandaria 
The Burning Crusade 
Wrath of the Lich King Addon Details
Expansion
Categories
Developers
Having issues?
Basically, enemies in WoW decide who to attack by deciding who is the most threatening based on the abilities you use. What Omen does is provide accurate values of your group’s relative threat level on individual enemies, so that you can see when you’re in danger of pulling aggro
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Cataclysm
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