Bone Key

The Bone Key is a drop from the Dungeon Guardian. It's used to summon a Baby Skeletron Head pet that follows you until you cancel the buff icon, like other pets. It has not been determined if it has a use.

Because the Dungeon Guardian has an insanely high amount of defense and instant-kills any player, it is an extremely difficult item to get.

History
Do keep in mind that this item cannot be obtained if Skeletron has already been defeated, a good tip for defeating the dungeon guardian is doing an infinity symbol like motion while constantly moving and firing you weapon. A magic weapon like vilethorn and other multi hitting magic weapon is a good idea to have around when fighting the Dungeon Guardian.