Skeletron

"The cursed guardian of the dungeon..."

Skeletron is a Pre-Hardmode boss. It must be defeated prior to entering the Dungeon, and defeating it grants unlimited access to all Dungeon areas for all players. It also causes the Clothier NPC to spawn, as long as a vacant House is available.

If attempting to enter the Dungeon before killing Skeletron, Dungeon Guardians spawn once a player reaches 0 depth. These enemies generally kill any player with a single hit, dealing 1000 / damage. Managing to defeat a Dungeon Guardian does not grant access to the Dungeon, thus defeating Skeletron is mandatory for entering the Dungeon.

While Skeletron is alive, music Boss 1 will play.

Summoning
Skeletron is summoned by speaking to the Old Man at the Dungeon's entrance at night, who will turn into Skeletron when selecting the “Curse” option. If the Old Man is spoken to during the day, he will ask the player to return at night. Skeletron can only be summoned once per night, as the Old Man will not respawn until dawn. While players with less than 300 maximum health and/or less than 30 defense are deemed too weak by the Old Man to lift his curse, he still offers the option to summon Skeletron.

Skeletron can be summoned repeatedly this way until it is defeated. Once it is defeated, the Old Man's curse will be lifted and he will no longer appear. The Clothier NPC will arrive, bearing a remarkable resemblance to the Old Man. He can move in a town, as long as there is an empty house for him to live in. Skeletron can be re-summoned thereafter once the player acquires the Clothier Voodoo Doll. Equipping the accessory allows the player to kill the Clothier, which summons Skeletron. However, this will only work at night. On any other occasion, Skeletron does not spawn.

Attack Phases
Skeletron is a large floating skull 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 attack phases: If the player takes too long to kill him and dawn arrives, Skeletron will begin acting exactly as 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). Skeletron remains in this form until dying or despawning, even if players manage to avoid dying until the following dusk.
 * 1) The head remains above while the hands slash at the player.
 * 2) The head spins, dealing significant damage if touched, but dropping its defense to zero for the duration of the attack.

Expert mode
Skeletron has additional properties in Expert mode:
 * Due to its high defense, Skeletron's Head is nearly invincible. However, after losing a hand, its defense drops, rendering it more vulnerable.
 * Once either of the hands are destroyed, the head begins to shoot homing skulls at the player which resemble Cursed Skulls. The frequency increases when Skeletron loses health or the other hand.
 * Skeletron moves faster and has better AI when targeting multiple players. With this, it is able to "stunlock" the player while spinning, able to almost instantly kill them if they aren't able to escape (e.g. by using a Grappling Hook).
 * Skeletron is able to inflict the debuffs Slow (from the hands) and Bleeding (from the head).

Tips

 * Do not jump onto Skeletron's head multiple times with the Slime Mount, as going too high will cause him to despawn (also, in Space the low gravity will make it almost impossible to get back down past him). However, inside of the Dungeon entrance the Slime Mount can be a valuable tool.
 * Going fast enough on a Minecart can cause the hands to despawn, leaving only the head to fight.
 * A full set of Fossil Armor combined with Grenades can take out both hands easily.
 * Using any ricocheting weapon (such as the Water Bolt found in the Dungeon) is extremely useful.
 * Spiky Balls can be a good idea to throw into the arena before spawning Skeletron.
 * With multiple players, it is a good idea to set the spawn point near the Dungeon, so that players can simply rejoin the fight if they die.
 * The Bee Gun, Beenade or the The Bee's Knees can quickly build a swarm of bees for constant damage.
 * Skeletron is immune to many debuffs, but not to Frostburn.
 * Attacking the hands first is advised as destroying them both will make it very easy to dodge Skeletron for the rest of the battle.
 * This is close to mandatory in, as Skeletron gains 100 defense as long as its hands are alive.
 * In Expert mode, if the player keeps moving in different directions, most homing skulls will miss; also note that they do not travel through tiles.
 * In Expert mode, it is a good idea to use a yoyo and an overhead barrier.

Trivia

 * Assuming that the player is at 5′ 9½″ (1.75 m) in terms of height, Skeletron's head would be about 11′ (3.35 m) tall and 8′ (2.43 m) wide. The hands would be 10′ (3 m) tall and 7′ (2 m) across.
 * – According to the official game lore, Skeletron is the Skull of Cthulhu.
 * At one point during game development, Skeletron Heads protected the Dungeon, but when one was killed, it would count as killing Skeletron, so the Dungeon Guardians were introduced to protect the Dungeon instead.
 * Counting the health from both its hands, Skeletron has even more health than the Expert mode Wall of Flesh.
 * Skeletron is the only boss not to have any guaranteed drops (with the exception of the Lesser Healing Potions and Treasure Bag if in Expert mode).
 * On the of Terraria, the Old Man has a health bar, but on the, he has none. However, this does not affect gameplay as there is no way to damage him legitimately.
 * In the Terraria and Edge of Space crossover teaser, Skeletron turns into an actual boss called Omegatron.
 * The name Skeletron is similar to the name, the main antagonist of the  franchise.
 * Blood emits from the joints between Skeletron's arms and head.

History
Skeletron Squeletron Csontkirály Скелетрон 骷髅王