Finally! Patch 3.3 introduced a handful of new mounts and one of them is probably one of the most interesting ever added to the game : Invincible.

Who's Invincible? Let's ask WoWWiki: Invincible was the steed of Arthas Menethil, both in life and death. He was the skeletal steed that Arthas rode throughout Warcraft III: Reign of Chaos and Warcraft III: The Frozen Throne.

Until a few builds ago I wasn't really sure if it was going to be a player mount or not but the addition of the Invincible's Reins feat of strength definitely confirmed that.
We do not know where it will drop but it's safe to assume that we'll have to defeat the Lich King in some crazy-hard difficulty mode. Also keep in mind that we won't fight him before months, it also means that players won't get this mount either and it could be modified before it hits the live servers. This is just a preview.