Dungeon Heist
Common40.9%ObjectiveOptionalAnywhere
Steal a key from the dungeon's undead denizens, and unlock one of their precious golden chests.
Post-Skeletron, dungeon enemies rarely drop a Golden Key; use it on any of the locked Golden Chests scattered through the dungeon (Muramasa, Cobalt Shield, Magic Missile and friends inside). Kill things near a locked chest until a key lands, unlock, pop. In 1.4 keys also come from dungeon slabs' pots and vases, so it's rarely a long wait.