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.

An easy method for killing the Guardian is using gravitation potions and a very fast-firing automatic weapon, such as the Megashark. As of 1.2.0.3, the Dungeon Guardian now deals less damage (reduced from 9000 to 1000), to avoid exploits using Turtle armor and a Black Belt or Master Ninja Gear.