Dungeon Guardian

The Dungeon Guardian is an enemy resembling Skeletron's head that guards the Dungeon against intruders. If a player enters the Dungeon in a world where Skeletron has not been defeated 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 damage each, which will instantly kill most players if they are unprepared. 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 (with very few exceptions), will deal only 1 damage, 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.

The Dungeon Guardian's Bestiary entry will automatically fill itself out if Skeletron is defeated.

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. Most of the following tips assume taking a late-game or endgame character to a fresh world where the Guardian can still be summoned, as they involve items that can only be obtained after defeating Skeletron (if not the Moon Lord).
 * However, on the, the Bone Key is required for the "Pet Hoarder" achievement.
 * In or  Dungeon Guardians will have   the health and damage without any exclusive drop. Due to this, it is recommended to try and defeat the Dungeon Guardian on a Classic world to save time, money (as all enemies take money from the ground), and further troubles.
 * High critical chance is crucial for fighting the Dungeon Guardian, as critical hits can tremendously speed up the fight.
 * 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 or Endless Quiver.
 * , as the Dungeon Guardian is not immune to the debuff it inflicts and it additionally ignores the defense, allowing it to deal 5 damage per tick.
 * Chain Gun, Megashark, or with Crystal Bullets or Endless Musket Pouch.
 * , which, while dangerous to use, inflicts the Bleeding debuff.
 * Piranha Gun for its good damage rate (7 against the Dungeon Guardian's defense) and lack of a need to aim. It also does not use ammo.
 * Zenith, due to its incredibly high swing rate and ability to hit anywhere onscreen. With appropriate melee accessories and armor, it can deal up to approximately 40–50 DPS in Expert Mode.
 * Since the Dungeon Guardian moves at 41 mph, having high mobility is essential. The (41 mph),  orUnicorn Mounts (61 mph) are suitable and many types of wings reach speeds above 41 mph, especially when the Soaring Insignia is combined with hovering wings. A Rod of Discord may help with turning around at the world's edge. Before 1.4.1.1, the Pirate Ship Mount traveled 82 mph horizontally, as opposed to the current 36 mph horizontally, which was especially effective at outrunning the Dungeon Guardian.
 * Frost armor is highly recommended for ranged and melee users because Dungeon Guardian is not immune to Frostburn debuff.
 * A good tactic is to equip full Hallowed armor /  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 and throw a boomerang, effectively preventing it from reaching the player while it will constantly be 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.
 * To defeat a Dungeon Guardian, the Depth Meter or its upgrades would be helpful when trying to figure out its spawn range, or else it will most likely catch the player off-guard.
 * In multiplayer, it can simply be lured out by another player.
 * Purchasing teleporters from the Steampunker NPC and laying them out apart from each other can prove itself to being a viable strategy.
 * By using the Portal Gun in a way in which the player is always teleporting will make it nearly impossible for the Dungeon Guardian to hit them.

Trivia

 * Originally, actual Skeletron Heads were spawning during the day, and they were extremely weak. Additionally, defeating all of them granted access to the Dungeon, skipping the actual Skeletron itself. The Dungeon Guardian was introduced to prevent this unintentional behavior.
 * Despite being initially intended to be unbeatable, it is not invulnerable like Spike Balls, Blazing Wheels, and True Eyes of Cthulhu.
 * It has no corpse on the versions, but on the, the remains of Skeletron's Hands will appear upon its death.
 * It uses Skeletron's old sprite from before 1.3.5.
 * Its head spins clockwise when the player is going left and counterclockwise when going right.
 * The Dungeon Guardian has the highest health of any non-boss and non-event enemy in the game.
 * Dungeon Guardian is immune to the Lightning Aura sentry summons' effect of ignoring defense.
 * In 2011, YouTuber Yrimir uploaded a video of him killing the Dungeon Guardian in, approximately, version 1.1. This prompted the developers to add the Bone Key as a reward for defeating it, as at the time, the Dungeon Guardian dropped nothing.
 * The Bestiary entry for the Dungeon Guardian: "A lethal sentry that stands watch over the Dungeon's gates. Those who are deemed unworthy to enter shall not pass!"
 * The end of the bestiary entry, “Those who are not worthy shall not pass!” may be a reference to, in which the character Gandalf says "You shall not pass!".