Dungeon Guardian

Dungeon Guardians are enemies that guard the Dungeon against intruders. If a player enters the Dungeon without first killing Skeletron and travels below zero depth (the horizontal midline of the map), multiple Dungeon Guardians quickly fly at the player, dealing 9,000 damage each, effectively killing any player instantly. Dungeon Guardians are large, very fast, and travel through blocks.

Dungeon Guardians are immune to the Poisoned, On Fire!, and Cursed Inferno debuffs.

Trivia

 * Originally, actual Skeletron Heads were used to guard the Dungeon. Killing all Skeletron Heads would count as defeating Skeletron, so the Dungeon Guardian was created to repair this unintentional behavior.