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 can also be noted that using the turtle armor will reflect damage back to the Dungeon Guardian, making it much easier to defeat. At least 2 people are required for this method, as the damage reflection does around half the Guardians health bar.