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, Daybroken and Shadowflame debuffs.

Tips

 * Because of the extreme effort that must be done to defeat a Dungeon Guardian, and also because its only drop, the Bone Key, isn't very useful, there isn't much point in defeating a Dungeon Guardian unless one wants bragging rights(or is bored, wants a challenge or practice at dodging).
 * Killing a Dungeon Guardian is possible, though it would take thousands of hits due to its absurdly high defense. (The Phantasm Bow and Lumanite Arrows are reccomended, because of the buildup that can easily drain his health by hundreds a second) 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, and every critical hit will do 2 damage.
 * Dungeon Guardians will adhere to the same spawn limits as any other enemy: if there are too many monsters on-screen at once, they will not spawn.
 * An efficient and fast 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 firing rate 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 Stardust 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.
 * The player can outrun the Dungeon Guardian with the Unicorn Mount. The Unicorn Mount's speed reaches 61 mph, faster than the Dungeon Guardian, so the player will need to stop to allow the Dungeon Guardian to catch up.
 * Use of a Life Drain is particularly effective against the Dungeon Guardian, as the Life Drain can deal 5 damage a tick.
 * Another good way to kill the Dungeon Guardian is to lure it into a circle of ropes, keep going around the ropes. Then, throw a boomerang in the middle. The Dungeon Guardian will stay in the middle, while taking damage from the boomerang.
 * However, the Guardian's speed will almost always overtake the player. This strategy is best done with a circular hoik. (see Travel for more details)
 * In the Chlorophyte Shotbow combined with Heart Arrow while wearing full Frost Armor (as the Dungeon Guardian is not immune to Frostburn debuff) can be a good way to defeat the Dungeon Guardian.
 * 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.

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.