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.

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, a Lifeforce Potion and having a friend with a Paladin's Shield equipped.
 * The Dungeon Guardian will not drop the Bone Key