Dungeon Guardian

The Dungeon Guardian is the guardian of the dungeon. If a player tries to break in the dungeon without killing Skeletron, this enemy will appear and kill him/her. In previous versions Skeletron Head was used instead of this.

Before the Guardian existed, killing all Skeletron heads was another way to kill Skeletron, so the Dungeon Guardian was created to circumvent this unintentional behavior.