Skeletron

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. Skeletron is a Boss enemy that guards a world's Dungeon. 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. If you enter the Dungeon before killing him, Dungeon Guardians will attack at 0 depth, dealing 1000 damage, effectively killing the player in one hit.

Skeletron is a massive floating skeleton head with floating arms. It will try to make contact with the player with his head and hands. 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. If the player takes too long to kill him and it turns to day, Skeletron will act like a Dungeon Guardian and kill the remaining players on the screen (note: it is still possible to kill him, but players are at an extreme disadvantage).

Skeletron alternates between two attacks. In the first one, his head will remain above and he will slash you with his arms. In the second one, he spins his head to attack you, dealing significant damage. However, during this second attack, his defense drops to 0.

Once you defeat Skeletron, you may enter the dungeon as many times as you like and the Clothier will move into a vacant House. You may also re-enact the fight against Skeletron by equipping the Clothier Voodoo Doll and killing the Clothier.

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

Trivia

 * Skeletron was the first boss to drop a mask, which was added in 1.2.
 * Although there is no relation, the Lego Movie happens to have a robot skeleton named Skeletron, much like the boss's hardmode counterpart.

History
Skeletron Скелетрон