Death knights are former paladins who have forsaken the Holy Light and entered the service of the Lich King.

The first of the Scourge’s death knights was Prince Arthas Menethil of Lordaeron, who succumbed to madness and entered into the service of the Lich King at the beginning of the Third War, after his soul was stolen by the runeblade Frostmourne.

Other once-noble paladins would follow in Arthas’ steps. During the fall of Lordaeron, the Order of the Silver Hand was disbanded, and its leader Uther Lightbringer slain. Some of the paladins traveled to the plague-ridden colonies, seeking to continue to help the people of the land. Though the Paladins were immune to disease of any kind, they were persecuted by the general populace who believed that they had been infected by the foul plague. Embittered by this rejection, a few traveled north to find the plague’s source.

Much as Arthas had before them, these renegade paladins succumbed to bitter hatred over the course of their grueling quest. When they finally reached Ner’zhul’s icy fortress in Northrend they had become dark and brooding. The Lich King offered them untold power in exchange for their services and loyalty. The weary, vengeful warriors accepted his dark pact, and although they retained their humanity, their twisted souls were bound to his evil will for all time. Bestowed with black runeblades and shadowy undead steeds, death knights serve as the Scourge’s mightiest generals.

In the new expansion of World of Warcraft, Wrath of the Lich King the player is able to become a Death Knight (Hero Class) through some kind of achievements/quests. (Full detail has not been given about the Death Knight Hero Class and the achievements/quests in order to obtain this soon to be playable Class at this time.)