Dungeon Guardian

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

Deep caves and Corruption chasms near the Dungeon should be avoided until Skeletron is slain, as merely being close to the Dungeon at a certain depth is considered a break-in.

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.