Apocalypse is the unholy death knight artifact weapon in Legion.
Long before the Burning Legion's third invasion of Azeroth, the nathrezim, otherwise known as dreadlords, forged the runeblade Apocalypse. Wherever it goes, violence, plague, and death follows. Apocalypse gained a horriic reputation after a Tirisgarde mage failed to control its energies. Eventually, the Black Riders came in possession of Apocalypse and stored it beneath Karazhan, where the blade was taken from by the Deathlord of the Knights of the Ebon Blade during the Burning Legion's third invasion of Azeroth.