Skeletron

Skeletron is the boss that guards a world's dungeon. If one enters the dungeon before killing him Dungeon Guardians will come spinning and growling into the screen, impacting with the player and causing 9000 damage, effectively one-hit killing the intruder. To summon this boss, you must talk to the Old Man at the entrance to the dungeon at night. The man will transform into the Skeletron. If you attempt this at day, he will simply say "Come back at night if you wish to enter" or "I cannot let you enter until you free me of my curse".

Skeletron is a floating, massive skeleton head, with floating arms. What appears to be blood spurting from his arms and neck are most likely "the magic forces holding it together", as noted by Blue in a let's play video.

Skeletron will try to swing his arms at you and has a special attack where he spins his head around and follows a player that should be avoided. His head, at 5000 health points and 10 defense, is the most resilient entity in the game with the exception of the Dungeon Guardians, so it is not a good idea for lower-end players to attempt to defeat him. Destroying the head also destroys any remaining arms that Skeletron has. He must also 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 on that map and must create a new one if you wish to fight him.

The old man who stands in front of the dungeon can be killed if you manage to deplete his health, however, if you enter the dungeon Skeletron will instantly spawn and kill you.

If you teleport to your home, Skeletron won't follow you. Also note that if a player fails to defeat Skeletron, Skeletron will instantly die without a "Skeletron has been defeated!" message.

If you build a small house and put a bed inside near Skeletron, it is possible to set your spawn point near the boss, making the respawns act as 'extra lives' against him. (Note: Only works on multiplayer)

As of Update 1.0.4 when you kill Skeletron a Clothier can move into a vacant house.

For more info, see Guide:Strategies for killing Skeletron.

Drops

 * 5-15 Lesser Healing Potions: 100% (1:1 chance)
 * Around 5