When Hades needs warriors to defend his realm or to serve his goals in the land of the living, he simply plucks a fallen warrior from the River Styx and sends them out to do his bidding. These warriors are single-minded, doing anything to achieve their mission. They are nearly impossible to best in battle. How do you slay something that is already dead?
During the Gorgon War, there were literally thousands of these warriors wading onto the battlefield. Whether they fought on the side of the humans or the Gorgons while they were living, they all became slaves to the whim of Hades and his goals once in his realm. At some point, they were all recalled to the underworld and no longer walked above. They could still be released at any time to carry out any of Hades' new plans.
The Undead Warrior comes with the following: