Bone Key

The Bone Key is a drop from the Dungeon Guardian. It is used to summon a Baby Skeletron Head pet that follows you until you cancel the buff icon, like other pets.

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

Do keep in mind that this item cannot be obtained in a world where Skeletron has already been defeated, as Dungeon Guardians will no longer spawn.