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 (the horizontal midline of the map), one or more Dungeon Guardians quickly fly at the player, dealing 1,000 damage each, effectively killing any player that they touch instantly. Dungeon Guardians are large, very fast, and travel through all blocks.

Dungeon Guardians are immune to the Poisoned, Venom, On Fire!, Cursed Inferno and Confused debuffs.

Tips
set your spawn near the Dungeon and summon a Dungeon Guardian let him attack you. Black Belts and Master Ninja Gear give you a 1/10 chance to dodge, and the set bonus from turtle armor will still apply damage even if you do dodge the attack, so you can kill the Dungeon Guardian without it doing any damage to you.
 * It is extremely easy to kill Dungeon Guardians in multiplayer on the Console version if you build two houses a bit away from each other and at least one person set their spawn at each house and with full turtle armor. You can kill a Dungeon Guardian in two or three hits since its health has not yet been upgraded, making easy Bone Key farming. A strategy for single player on Console with full turtle armor and a Black Belt or Master Ninja Gear is to

Trivia

 * The Dungeon Guardian's insane stats and difficulty causes some players to consider it a boss.
 * 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.
 * If you continue going down into the Dungeon, and manage to dodge the first Dungeon Guardian, more may appear and eventually, quickly kill you due to their immense speed.
 * It is possible to survive a Dungeon Guardian using Beetle armor (with shell), defense + warding accessories and Ironskin and Well Fed buffs and 500 life. The guardian may hit 500 hp, which will kill you in one hit. Sometimes he will hit somewhere in the 480s and you will survive, you can reduce this damage further using Endurance Potions and having a friend with a Paladin's Shield equipped.
 * On Mobile version (During San Valentine) the best way to defeat Dungeon Guardian is using Heart Arrows as it can stun it making it ridiculously easy. However buying these arrows may be very expensive (a stack of 10,000 arrows costs ) and is a must use the Necro armor, also it doesn't drop any loot making it wasteful.