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.

Any player attack on a Dungeon Guardian, regardless of the weapon or other equipment, will deal only 1 damage, or 2 for critical hits.

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.
 * Due to being invincible to direct damage in the tutorial on console, it is possible to survive all attacks from the Dungeon Guardian. This, however, is not the best way to defeat him as one; you will be knocked around incessantly without a cobalt shield making moving a challenge and two; the absurdly high defense of the Guardian makes it nearly impossible to kill him. If caught in this situation, the best way to stop is to drown yourself or be poisoned until you die.
 * You can trap (the) Dungeon Guardian(s) into an area and then they won't move so you can get (a) Bone Key(s)! (confirmed a glitch).