The Undead Army
Except for the obvious differences (lack of skin, hair, a working circulatory system, etc.) the undead army are very much like any living human army. They have military ranks, a chain of command, and a set of marching orders. The bulk of the army is made up of once-live humans. However, there are tales that when this force marches into battle, the animated skeleton of a once-grand dragon marches with it.
There is much speculation about the undead of Tyria and what their goals might be. Many believe that they are the remnants of Orr, those lost souls who were so disturbed by the destruction of their home in the Cataclysm that they refuse to pass into the next life. Others believe there is a more sinister force motivating these creatures, a malevolent being who has raised the resting dead to do his bidding. Some even say this mysterious leader is a powerful lich lord who rules them all from behind the front lines. But most of this is just campfire tales and barroom gossip, for there is no one who lives today who can claim to have seen this creature in the flesh (so to speak).