The Death Knight is a powerful undead monster originating from Dungeons & Dragons. Taking the form of a heavily armored undead warrior with potent magical abilities, it is something of a more gishy Lich and has become an iconic "upgrade" for the Blackguard.
In 4th edition, the similarity to liches was emphasized by giving Death Knights the same ability to reconstitute themselves after death, with their weapons serving as a kind of phylactery.
The Pathfinder Graveknight is much the same creature, with the dead warrior's soul bound to his armour in much the same way as a Lich's is bound to a phylactery. This tends to go badly for anyone who loots a defeated Graveknight's armour, with the wearer being transformed into a replacement body