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

 * Using a long hall of spiked ball traps, with teleporters on each side can be an effective way of killing the Dungeon Guardian, as shown in this video: https://www.youtube.com/watch?v=ZepP3r_piLs