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 (a horizontal border separating the surface from the underground), one or more Dungeon Guardians quickly fly at the player, dealing 1,000 damage each, which effectively kills most players instantly. Dungeon Guardians are large, very fast, and travel through all blocks.

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