An undead knight whom can use some necromantic magic. Similar to how a lich has it's soul contained in a trinket, a Death Knight's soul is contained in their armor or sword. As long as the item is intact, the Death Knight has the same technical invincibility as a lich-they can be defeated, but their body regenerates inside their armor or holding their sword. The item must be destroyed utterly; simply breaking the armor or sword won't work. Dissolving the peices one by one in acid or lava might. Only the most cruel of warriors become death knights.