"How do you know which one is real?!" —Dizdemona[1]
Mirror Image is a level 45 mage talent. Casting it creates three copies of the mage that cast spells at their current target and have a fraction of their health and intellect.
The Mirror Images inherit the mage's threat lists, and the mage also gains a 90M threat reduction debuff like [Fade]. This makes it useful as a last-ditch threat dump.
Any players targeting the mage when the spell is cast will find themselves targeting one of the images instead, and may waste valuable attacks on the image, or valuable seconds searching for the real mage. This makes it a very useful escape technique in PvP.
Mirror Images now inherit 100% of the mage's Spell Power (up from 5%), last 40 seconds (up from 30 seconds), and have a 2-minute cooldown (down from 3 minutes).
Patch 5.3.0 (2013-05-21): Created copies no longer cast [Fire Blast], but their Frostbolts do 10% more damage.
Images will no longer trigger the death sound when their time expires.
The mirror images will no longer complete casts of Frostbolt on targets which are Polymorphed at the time their Frostbolt channel finishes.
Patch 3.1.2 (2009-05-19): The images summoned by this spell will now target the creature that most hates the Mage, and should no longer cast [Fire Blast] or [Frostbolt] on targets that are affected by crowd control debuffs that break immediately on damage unless they are already casting these spells when crowd control is applied.
[Arcane Power] or 
[Mastery: Mana Adept] further increases their damage as well as their master's 
[Arcane Blast] stacks. Mirror Images inherit 1/3 of their master's spell power each.
