Have the expansion The Cataclysm activated along with its characters. Play as the Scavenger. Start a game, and you'll see a list of five cards. If the Cataclysm Blade is among them, keep playing. If not, exit and start a new game, doing so over and over until it happens. Then when you roll a 1 to move, keep choosing to Scavenge instead until you pull the Cataclysm Blade. The achievement then unlocks.