Skeletron

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Skeletron is a Boss that must be defeated before traveling through the Dungeon. He is immune to the Poisoned, On Fire!, and Cursed Inferno debuffs.

Defeating Skeletron grants unlimited access to all Dungeon areas for all players. It also causes the Clothier NPC to spawn, so long as a vacant House is available.

If you enter the Dungeon before killing Skeletron, Dungeon Guardians spawn once you reach 0 depth, generally killing any player with a single hit.

Summoning
Skeletron is summoned by speaking to the Old Man at the Dungeon's entrance, at night, who will transform into Skeletron when you select his "curse" option. If you speak to the Old Man during the day, he will ask you to return at night.

Skeletron can be summoned repeatedly this way until defeated. Once he is defeated, the Old Man will no longer appear. Skeletron can be re-summoned thereafter once the player acquires the Clothier Voodoo Doll. Equipping the Doll allows the player to kill the Clothier, which summons Skeletron.

Attacks
Skeletron is a large floating skeleton head with two independent floating hands. All parts can travel through blocks, and will attempt to make contact with the player to inflict damage. Each part can be killed independently, but killing the head is all that's required to defeat Skeletron.

Skeletron alternates between two attacks:
 * The head remains above while his hands slash at the player.
 * The head spins, dealing significant damage, but its defense drops to zero.

If the player takes too long to kill him and dawn arrives, Skeletron will act like a Dungeon Guardian, performing a head spin attack with very high damage, which will usually kill any players remaining in the vicinity quickly (it is still possible to kill him, but players are at an extreme disadvantage).

Trivia

 * Skeletron's head is the most resilient entity in the game prior to the Wall of Flesh and Hardmode bosses.
 * Skeletron was the first boss to drop a mask, which was added in 1.2.

History
Skeletron Скелетрон