Skeletron

 is a boss which can be 'summoned' by talking to the Old Man in front of a Dungeon at night. If you choose 'curse' when talking to him, he will immediately transform into Skeletron and start attacking you, so be prepared when this happens. This is a difficult boss, do not try to fight him unless you have lots of potions, good pre-hard mode equipment and/or throwable objects. It is also notable that Skeletron is the only boss that can only be summoned and defeated once per world.

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When you defeat Skeletron, you will gain access to Dungeons, and the Clothier NPC will come to your town if you have a house for him.

Attacks
Skeletron will attempt to hit you with his arms by swinging them at you and will sometimes spin his head, put his arms in the air, and chase you with his head. He also often slams his arms together, attempting to crush you. If you take too long to kill him, and the sun comes up the next day, Skeletron will hit you for 9999 damage (Minus defense), killing you instantly.

Tactics
There are a few things to keep in mind while fighting this boss with any technique.

First, try to keep the fight on a nice flat surface. It makes Skeletron's attacks far easier to dodge and makes attacking him just as easy. Secondly, watch out for his hands, as they will usually hit you the most often. Lastly, always watch for when he rolls his head towards you. while attacking like this, he is extremely vulnerable, and you can even do quite a bit of damage with swords such as the Blade of Grass. Also, start your fight at the earliest possible because you need to defeat him before daybreak.

One technique (and probably the most common) is to use ranged weapons. Simply spamming the ranged weapon at Skeletron's head while dodging his attacks is usually quite effective, especially if it's rapid fire. Good weapons to use would be the Phoenix Blaster, Sandgun (if you don't mind sand getting covered all over the surrounding area), Minishark, Star Cannon (assuming you can afford the strange form of ammunition ), and the Space Gun. The Molten Fury is also a good choice. Shuriken and Throwing Knives are a possible but challenging way to kill Skeletron earlier in the game.

Another technique to better conserve ammo is to use ranged weapons to attack Skeletron's hands first, and then deal with the head using swords that can hit behind you, flail weapons or weapons that return to you, like the Flamarang.

Another easy technique is using Spike Balls at the arm, which will destroy Skeletron's arms in a matter of seconds because Spike Ball is exceptionally strong. After its arms are destroyed use any ranged weapon such as bows or Thorn Chakram which will finish of the Skeletron's head. You also may use spike balls in order to kill it faster, I, Badass366 used this technique on Eater of Worlds, which killed it in a matter of seconds.

Tips
It is recommended that you do not wear Necro Armor for this fight, as it changes the sound you make when you are damaged to same one that Skeletron makes. This can result in you not noticing when you take damage, especially if you're using the Cobalt Shield. One solution to this problem is to wear something else in the Vanity Slot, as this prevents your damage sound from changing. During the fight, if you don't have a Minishark, or another powerful weapon, it is useful to take down his arms/hands. After this, it will just be his head that you will deal with, thus making the fight easier since you will just simply jump and not be damaged.

Dungeon Guardian
If you try to go into the dungeon by going through the main entrance or by digging in without defeating Skeletron first, a Dungeon Guardian (which looks like a spinning Skeletron head) will ram into and kill you along with any other players who may be nearby. There are now several videos of people beating him. There's nothing, no death scream, no loot, no corpse after you defeat him. So you can kill him, but that can take about 15 minutes and is very difficult without proper preparation.

The Dungeon Guardian only appears when you move in front of the dungeon's back wall below rock level, even if you placed it. Using a Depth Meter can help in finding the exact level. Destroying the back wall with a hammer will prevent the Dungeon Guardian from spawning although being hit by other dungeon mobs will usually knock you over walls you have not destroyed unless you have a Cobalt Shield equipped. One way to help destroy background tiles is to use Dynamite.

Bugs

 * An interesting graphical bug appears when you fail to kill Skeletron but survive (if he flies too far off screen) and see the Old Man spawn, when you kill Skeletron, He will drop multiple headgibs, each dropping its own items. Twotrons.jpg
 * If Skeletron spawns too far outside the dungeon, he will spawn with only a head.
 * Rarely when you spawn Skeletron, The Twins may spawn even if you're not on Hard Mode yet.

Trivia
As of 1.1, skeletron has a tougher, more intimidating variant known as Skeletron Prime. It has 4 arms, has a metallic look and its head is covered in spikes.