Mirror Image is a core mage ability learned at level 49. Casting it creates three copies of the mage that cast spells at the current target and have a fraction of the mage's health and spell power. This ability was added in Wrath of the Lich King.
Use: Permanently teaches you this glyph. Your Mirror Images cast Arcane Blast or Fireball instead of Frostbolt depending on your primary talent tree.
Sell Price: 1
This spell is on the global cooldown, making its activation a slight DPS loss. However, it does not trigger a GCD on every other mage spell that doesn't have a global cooldown such as Cold Snap, Icy Veins, Combustion, and damage trinkets.
The images created do low damage and receive a percentage of the mage's spell power. They cast 3.0 second Frostbolts. Mirror Images inherit the spell hit rating on their master's gear (but not that of talents).
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.
The images do an average of 15k damage over 30 seconds with no spell power, which is highly useful for single-target DPS in raids and sustained PvP burst.
Patch 5.3.0 (20-May-2013): Will now chain cast spells with no delay between casts, allowing images to cast their spells more frequently in the same amount of time.; Mirror Image creates copies that no longer cast Fire Blast, but their Frostbolts do 10% more damage.
Patch 5.2.0 (5-Mar-2013): Frostbolt cast by Mirror Images now deals 50% more damage.
Patch 3.3.0 (08-Dec-2009): Health on the mirror images has been increased. The mirror images summoned will no longer sometimes appear as shades before taking on the appearance of the casting mage.
Patch 3.2.0 (04-Aug-2009): 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.