Possessed Skull

Skeletron Jr. floats along behind you and starts whirling its skull to close the gap whenever you jump or pull ahead, the same spin the full sized version uses when it chases you down. It does no damage and gives no bonus. It is a trophy you carry around.

Master Mode only, from Skeletron himself, and not on every kill. That is less painful than it sounds because he is cheap to re summon with a voodoo doll farmed off dungeon casters and angry bones, so repeat attempts are just dungeon trips rather than waiting on anything.