Riding on the chill winds that flow around the frozen spires of Icecrown Citadel comes the once-noble steed of Prince Arthas Menethil. Now this beast, raised from its grave, bears the fearsome Lich King on its winged back. Even in undeath, it remains an ever-faithful servant. Learn more of its tainted history here.

http://www.worldofwarcraft.com/info/underdev/3p3/invincible.xml

• Invincible will only be available in the 25-person encounter with the Lich King.

• Much like Mimiron’s head, one lucky player will be able to attain this incredible mount when the Lich King is defeated in his strongest form

(Source)