Aegwynn resurrected him. Come to think of it, is that why he became all mysterious and prophet-y after he returned from the dead? Did he get some cool insight from the Shadowlands while he was there? What Aegwynn did wasn't necromancy, she brought back a guy who was decapitated and buried for years in a new flesh and blood body. Would be cool if there was evidence of her knowing about and manipulating the Shadowlands.
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Honestly surprised the first Death Knight didn't show up in Maldraxxus. Maybe he is buried in one of those crypts in Revendreth, or maybe he was bad enough to go straight to the Maw.