Skeletron

Skeletron is the boss that guards a world's dungeon. If one tries to enter a dungeon before killing Skeletron, after a short moment the head of the boss will come spinning and growling into the screen, impacting with the player and causing 9999 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 and talk to me at night".

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.

Skeleton will try to swing his arms at you and ram you. He has 6000 health points, so it is not a good idea for lower-end players to attempt to defeat this monster. 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, with no bother from skeletron anymore. Once you defeat him, you may not battle him again on that map, and must create a new one to fight him.

Currently, if you attempt to enter the dungeon at night without defeating Skeletron, they will do 35 damage like the boss. Meaning you could theoretically run into the dungeon and loot a few chests at night if you had potions. If you manage to defeat one (or more) of the Skeletron heads, it will count as defeating the boss, unlocking the dungeon normally.