The Traitor
Rare7.8%ObjectiveOptional
Defeat any boss by playing one of his own minions.
Beat a boss while playing as one of that boss's own minions. Easiest route: unlock the playable Barbarian by clearing the Barbarian Arena (survive all ten waves), then start a game as the Barbarian and kill the Barbarian Boss. Other pairings work too (Snakey vs Medusa, Iceskimo vs the Frost King).