Blizzard have once again updated the Warcraft III Web Site with a new unit, The Death Knight, which is one of the undead heroes. Here is a description:

Death Knights were once virtuous defenders of Humanity. However, they were corrupted by the subtle machinations of the Lich King and lured to his standard. These former heroes were given untold power and the promise of immortality in exchange for their loyalty. Although they retained their humanity, their twisted souls were bound to the Lich King's will for all time. Bestowed with black, vampiric Runeblades and shadowy steeds, Death Knights now serve as the Scourge's mightiest generals.