Evil gods, demon lords, and various malevolent beings will sometimes grant undeath and powerful abilities to evil Paladins who have died. Some Paladins seek the powers granted by these divine beings so badly, that they willingly perform a ritualistic sacrifice upon themselves. Death Knights look similar to how they did in life, though with visible bones and/or rotting flesh. Death Knights often ride Nightmares into battle and use a variety of undead servants, like skeletons, wights, and zombies.
Death Knights are well established in many official campaigns, such Lord Soth of Dragonlance and Ravenloft, Prince Moren of Eberron, Miltiades and Olanthius of the Forgotten Realms, Prince Moren of Eberron, and Saint Kargoth of Greyhawk.
Not all Death Knights are necessarily evil by choice. Some were Paladins who had fallen from grace and cursed by their own deities as punishment. Some Death Knights have redeemed their souls or atoned for their actions, and had this curse lifted. They typically serve more powerful beings, but unlike most undead, Death Knights retain autonomy over their own thoughts and decision-making.
- from the publisher's blurb