Skeletron

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. Skeletron is a Boss enemy that guards a world's Dungeon. If you enter the Dungeon before killing him, Dungeon Guardians will attack, causing 9999 damage, effectively killing the player with one hit. To summon this boss, you must talk to the Old Man at the entrance at night, who will transform into Skeletron. If you attempt during the day, he will ask you to return during the night.

Skeletron is a massive floating skeleton head with floating arms. It will try to make contact with the player with his head and arms. His head has 4,400 health points and 10 defense, and is the most resilient entity in the game prior to the Wall of Flesh and Hardmode bosses. Destroying the head kills Skeletron. He must be defeated before the break of dawn; all players will instantly die if he is still alive at that point.

Once you defeat Skeletron, you may enter the dungeon as many times as you like. You may not battle him again in the same World once defeated, and must create a new world if you wish to fight him again.

When you kill Skeletron, the Clothier will move into a vacant House.

Skeletron is immune to the Poisoned and the On Fire! debuffs.

Video Example
http://www.youtube.com/watch?v=oR9MVi90Nho

History
Skeletron Скелетрон