Shadowmourne is a legendary two-handed axe which is a quest reward from Neutral 15. It is available exclusively to warriors, paladins, and death knights.
More to the point, this is a freakin' cool looking axe wot should be on your wall. Seriously this thing is MASSIVE, and detailed and did I mention just how COOL this thing is? It has two skull thingies so that's gotta be good right? And a blade with sticky out bits to catch someones soft bits and do untold damage to them - nice!