Dungeon Guardian

The Dungeon Guardian is an enemy that guards the Dungeon against intruders. If a player enters the Dungeon without killing Skeletron first and travels below zero depth (a horizontal invisible border separating the surface from the underground), one or more Dungeon Guardians quickly fly at the player, dealing 1000 / damage each, which effectively kills most players instantly. Dungeon Guardians are large, very fast (traveling at around 41 mph in all directions), and travel through all blocks.

Any player who attacks a Dungeon Guardian, regardless of the weapon or other equipment, will deal only 1 damage ( with the exception of Frostburn items, which deal 32 damage per second with the debuff), or 2 for critical hits. This means that the Dungeon Guardian will normally take an average of 1.04 damage.

While killing a Dungeon Guardian is possible (and will award the Bone Key pet summon), it requires careful preparation. See the Tips section for setups. Note that it does not grant access to the Dungeon.

Tips

 * Because of the extreme effort that has to be taken to defeat a Dungeon Guardian, and because its only drop, the Bone Key, is a purely cosmetic item, defeating a Dungeon Guardian is pointless from a practical perspective.
 * However, on the, the Bone Key is required for the "Pet Hoarder" achievement.
 * In, 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, money (as all enemies take money from the ground), and further troubles.
 * Weapons with a high rate of fire or splash damage are preferable, in order to compensate for the reduced damage. Viable options include:
 * with Luminite Arrows
 * , as it can deal 5 damage per tick
 * Chain Gun, Megashark, or with Crystal Bullets
 * , which, while dangerous to use, additionally inflicts the Bleeding debuff
 * Chlorophyte Shotbow or Tsunami paired with Heart Arrows (additionally inflict the Stunned debuff) wearing a full set of Frost armor
 * High critical chance is crucial for fighting the Dungeon Guardian, as critical hits can tremendously speed up the fight.
 * Since the Dungeon Guardian moves at 41 mph, having high mobility is essential. The (41 mph) and Unicorn Mounts (61 mph) are suitable, and a Rod of Discord may facilitate turning around at the world's edge.
 * A good tactic is to acquire full Titanium armor, allowing to occasionally dodge otherwise lethal hits.
 * Another good way to kill the Dungeon Guardian is to lure it into a circular hoik, throwing a boomerang in the middle. The Dungeon Guardian will stay in the middle while taking damage from the boomerang.
 * In order to quickly reach the surface after spawning the Dungeon Guardian, it is recommended to dig a shaft into the Dungeon, flying up as soon as reaching zero depth.

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 initially intended to be unbeatable, it is not invulnerable like Spike Balls, Blazing Wheels, and True Eyes of Cthulhu are.
 * It has no corpse on the versions, but on the  versions, the remains of Skeletron Hands will appear upon its death.
 * Its head spins clockwise when the player is going left and counterclockwise when going right.
 * The Expert mode Dungeon Guardian has the highest health of any non-boss and non-event enemy in the game.
 * In 2011, Yrimir uploaded a video of him killing the dungeon guardian in ~1.1. This prompted the developers to add the bone key as a reward for defeating it, as at the time, killing the dungeon guardian yielded no loot.