Dungeon Guardian

The  is an enemy that spawns if the player enters the dungeon in a world where Skeletron has not been killed. It will appear in order to kill the player as soon as the dungeon music starts. In addition, it will also target any other players in the vicinity of the player who entered the dungeon.

Due to its unattainably high attack and defense, the Dungeon Guardian will always take 1 damage from all attacks (except criticals, which deal 2 damage) and will kill any player in one hit.

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History
Before the Dungeon Guardian existed, Skeletron Heads fulfilled a similar role. Unlike with the Dungeon Guardian, killing all of the Skeletron Heads after they appear will grant the player access to the Dungeon as if they had killed Skeletron normally. The Dungeon Guardian was presumably introduced to counteract this unintended behavior.

Strategies
Due to the high defense of the Dungeon Guardian, it is best to use armor/accessories that boost your critical strike chance rather than damage, and use a rapid hitting weapon such as a Megashark with Crystal Bullets.

One tactic that can be used to kill the Dungeon Guardian (post 1.2) is to make a skybridge out of Asphalt Block. Run on the asphalt with Hermes/Flurry/Spectre/Lightning Boots in order to maintain a faster speed than the dungeon guardian. Get four Teleporters and set them one on each end and then an  other 60-70 blocks away from the one nearest to the edge. Put a pressure plate on each of the edge teleporters and wire them so that once you step on it, you teleport in the opposite direction.

This will allow you to switch running directions without being killed by the guardian. If using a Minishark or Megashark, this fight should take you no longer than 10-15 minutes. - Designed by Rix55

You can also use the Piranha Gun and Frost Armor, since the dungeon guardian is affected by frostburn and the piranha gun latch on the dungeon guardian, make it easier because you don't have to aim, however, you need to be close, else the piranha gun will return and need to be fired again.

Another tactic is to wear Turtle Armor and you also need to wear the Black Belt. When the Dungeon Guardian hits you, it also takes full damage. When the Dungeon Guardian hits you twice, you will kill it. The Black Belt gives you a chance of dodging an attack, which means that you won't get hit. There is a very small chance that you can dodge an attack twice, and kill the Dungeon Guardian. This tactic is not advised though because you will have to be very lucky! As of the 1.2.3 update, due to the Dungeon Guardian's attack reduced to 1000 (from 9999), this method might not be as effective. If you get 1 dodge,  the fight against Dungeon Guardian would be facilitated a lot

Another tactic is to use venom bullets and a mega/mini shark, since he isn't resisistant to venom, this makes the fight go way faster becuse he always takes 4 hits per second. And is you combine this with the frostburn bonus of the ice armor set, the fight will go really quick

Bugs

 * When wearing a set of Titanium armor, and having atleast 9 or 10 buffs, after killing an enemy, the armor will give you a debuff called "Shadow Dodge". Yrimir said that if you have met these requirements, you will be in godmode. This is good when fighting Dungeon Guardian...