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 who attacks 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, Confused, Penetrated, Celled and Daybroken debuffs.

Tips

 * Killing a Dungeon Guardian is possible, though it would take thousands of hits due to its absurdly high defense. Combined with the fact that you generally need to contend with multiple Guardians at once, victory is extremely unlikely unless you prepare carefully for the encounter.
 * High speed and critical chance are crucial for fighting the Dungeon Guardian, as every hit will do 1 damage, or 2 damage if it is a critical hit.
 * Dungeon Guardians will adhere to the same spawn limits as any other monster: if there are too many monsters on-screen at once, they will not spawn.
 * An efficient way to defeat the Dungeon Guardian is by using the Phantasm with the Endless Quiver. Although each hit will deal 1-2 damage, the high fire speed and additional homing arrows provide enough damage to deal about 75 damage per second on average. This combined with the UFO mount and the Stardust Cell or Dragon can take out Dungeon Guardians in about 5 minutes. This time can be lowered if you do not miss any arrow shots even when turning at the world border.
 * In Expert mode, Dungeon Guardians will have double the health and deal double the damage. Due to this, it is recommended to try and defeat the Dungeon Guardian on a normal world to save time and further troubles.
 * You can out run the Dungeon Guardian with the Unicorn Mount. The Unicorn Mount's speed reaches 61 mph, faster than the Dungeon Guardian, so you have to stop continuously so the Dungeon Guardian can catch up.
 * In the Chlorophyte Shotbow combined with Heart Arrow can be a good way to defeat the Dungeon Guardian.
 * Use of a Life Drain is particularly effective against the Dungeon Guardian, as the Life Drain can deal 5 damage a tick to the guardian.
 * Because of the extreme effort that must be done to defeat a Dungeon Guardian, and also because it's only drop, the Bone Key, isn't very useful, there isn't much point in defeating a Dungeon Guardian unless one wants bragging rights.

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.
 * Despite being intended to be unbeatable, it is not invincible like Spike Balls, Blazing Wheels, and True Eyes of Cthulhu are.