Bone Key

The Bone Key summons a Baby Skeletron Head pet that follows you until you cancel the buff icon, like other pets. It is dropped by Dungeon Guardians. Because Dungeon Guardians have insanely high stats intended to instantly kill most players, this is an extremely difficult item to acquire.

Tips

 * It's likely the player will need some sort of trap mechanism, such as a long hall of spiked ball traps with teleporters on each side, to defeat a Dungeon Guardian and obtain a Bone Key.
 * If you have wings, you can dig a hole in the dungeon deep enough for the guardian to spawn, and then fly up to the surface then encounter it.