Skeletron Prime

 is the Hard Mode equivalent of Skeletron. It is comprised of a head and four arms, each with different HP, power, defense and attack patterns. Just as Skeletron is the hardest pre-hardmode boss, Skeletron Prime was considered the hardest boss in the game. In all, he has 25,000 health.

Skeletron Prime will sometimes spawn at night with the message "You feel the air getting cold around you." appearing the previous dusk. However, he will refrain from spawning as long as the player is in the Underworld. The instant the player emerges upon the surface, he will spawn. Should this cause him to spawn during daylight, the player will most likely be killed instantly.

When killed, it will drop about 20-40 Souls of Fright, 14 Gold Coins, and 5-15 Greater Healing Potions. He will also drop 20-30 hallowed bars and has a rare chance to drop a Skeletron Prime trophy.

If all players are killed, Skeletron Prime will disappear underground. The only way to see him again is to summon him.

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Overview
Skeletron Prime is comprised of five individual parts: the head and four different limbs. This makes it the most complex monster in Terraria yet.

Skeletron Prime
This is the part with the most HP and is also the main part of the boss &#8212; when this part is destroyed, Skeletron Prime is defeated. Its attack pattern is the same as Skeletron: it will usually keep its distance from the player, but will periodically start spinning and lunge at them for increased damage and defense. However, unlike Skeletron, it grows spikes from its head when attacking, which give it a slightly larger range, so care must be taken to not get too close.

Prime Laser
The Prime Laser orbits around the head, shooting a low-damage beam at the player every time it is facing them. With 5,000 HP and a low amount of defense, it is the least threatening of the boss's weapons, although it is not to be underestimated. It will sometimes lock on to the player and shoot several beams in succession, so active dodging is recommended.

Prime Cannon
The Prime Cannon is slightly more resistant than the Laser (6,000 HP) and will periodically shoot prime bombs that deal average damage to a player (about 30 per shot with a full Adamantite set). Depending on its own state and that of the other limbs, it will start replacing the single shots with volleys of 3-5 cannonballs, so it is best to keep moving, especially when the boss is directly above the player. The bombs inflict the Debuff "Confused" and other ailments.

''Note: The cannonballs only explode on contact with a solid block, and will pass straight through Wood Platforms, so standing on an aptly-built line of them will render this weapon useless. Using a form of flying (Gravitation Potion, for example), will also allow you to avoid the cannon's balls.''

Prime Vice
The Prime Vice is Skeletron Prime's main melee weapon. When near the player, it will make a lunge at them. It has 8,000 HP and greater defense than the ranged weapons, and also has a rather large hitbox, so it will likely intercept part of the bullets shot at the head.

Prime Saw
The Prime Saw is the boss's most dangerous weapon. It has 8,000 HP, the most defense of all limbs, and its attack is a lunge at the player that, when it connects, scores a rapid streak of powerful hits. As long as the Prime Saw is on the field, extra care should be taken in order to prevent getting too close, lest the player quickly lose a lot health to this attack. A Cross Necklace can be used to lessen the damage done by this weapon due to the fact that it increases the duration of invincibility after being attacked.

Pointers and Behavior

 * Unlike most summonable bosses, Skeletron Prime will not despawn at dawn, but (just like Skeletron) will gain a huge defense and offense buff and will start to relentlessly chase the player (pretty much acting like the Dungeon Guardian), ultimately killing them in one blow. It is therefore best to summon him just after sunset (7:30 PM in-game)
 * Skeletron Prime will react to most attacks by backing away from the player, often off-screen so using fullscreen mode with a large resolution is recommended.
 * The Vice and Saw will briefly detach from the body whenever they attack the player: this is a good chance to score some hits at the head without the risk of the shots being blocked.
 * Like Skeletron, the hands' hitbox doesn't include the arms, so it is safer and simpler to just jump over them rather than completely move out of the way.
 * Avoid using explosives like Hellfire Arrows or Grenades; a melee attack could suddenly block the projectile and the splash damage could hurt you badly.
 * Each part of the boss can target a different player; This allows a team of players to split them up, causing them to spread out and become less effective.
 * Be careful using rapid-Mana costing weapons as you can run out of Mana very quickly. Always have an alternative like far shooting melee weapons (like Vampire Knives) or ranged weapons when you do.

Strategy

 * Since the battle can drag for quite a bit, it is advisable to wear the best pieces of armor available, since Skeletron Prime has a huge arsenal of attacks, it is likely that the player will take many hits during the battle, so maximizing defense should be the main priority.
 * Bring powerful ranged weapons; the Megashark will deal lots of damage quickly, while the Hallowed Repeater will let you exploit the buff from Archery Potion and use the special attacks of several arrows to quickly deal with all the parts. If you have Vampire Knives  use them, the healing bonus will be very helpful during the battle.
 * If you have the materials, Try using Full set of frost armor, Golden Shower, 5-15 greater healing potions, and a megashark. This technique will help you fighting Skeletron prime alot. First use the golden shower till your mana is depleted, then shoot at Skeletron's head with a megashark, once your mana has refilled, attack it with the golden shower again. continue to use this this technique, and you should be able to kill up to 3 skeletron primes in one night.
 * For a magic approach, use a full set of Spectre Armor  and rapid-firing, area spells like the Shadowbeam Staff, Golden Shower, Cursed Flames, Unholy Trident or Inferno Fork.
 * Other than attacking the player and blocking their way, the hands will protect the head against any non-piercing weapon: Holy Arrows, Crystal Bullets and/or the Cursed Flames spell are a good way to pierce through its defenses and damage the head and arms simultaneously. A Star Cannon can deal large amounts of damage, but would require a large amount of ammunition to be a viable option. Flails are a poor choice, because the boss will always try to gain higher ground, and gravity will greatly reduce the flail's vertical range (Unless it's directly above the player). Clouds placed high enough with a Nimbus Rod will continuously damage all parts of the boss.
 * Since Skeletron Prime tends to move back and forth relative to the player, it is best to stand ground in one place (preferably fitted with a heart/star generator) and go after the boss every time it is in range, then coming back down and waiting for it to descend again. Rocket Boots and Wings greatly aid this strategy.
 * Once the boss is down to one or no weapons, the head can be easily finished off with a spear or flail, since they can score several hits per use and the head's movement is predictable enough.
 * All of the parts can be afflicted with On Fire and Cursed Inferno debuff, so using a Flamethrower or Cursed Flames will deal continuous damage to them even when not under direct attack, though the Flamethrower will generally not be available the first time fighting this boss, as it is crafted from Souls of Fright.
 * If you don't want to make an arena, you could also try to fly around with Gravitation Potions and Wings while shooting Skeletron, which also makes it easier to visit the Nurse when necessary without risking her death. Summon Skeletron and fly around in a dodgy pattern, NEVER fly downwards (unless you are visiting the Nurse), and occasionally heal up via the Nurse.
 * Attack in this order: cannon, laser, saw, vice, head. it makes it easier
 * Summons from Hard Mode weapons, like the Ice Hydra, Pygmies and Leaf Crystal, are a great addition to your DPS.
 * Since you can acess chlorophyte before killing Skeletron Prime, you can get chlorophyte armor, make yourself some chlorophyte armor (with headgear), a chlorophyte shotbow (shoots 2-4 arrows from one) and chlorophyte arrows. Since chlorophyte arrows can pierce, and bounce on walls, the hands and arrow misses won't be too much trouble. Also the leaf crystal makes the fight faster and easier.

Trivia

 * Skeletron Prime has a head and hands made of metal, while its arms remain made of bone.
 * The name seems to be a reference to Megatron and Optimus Prime from the Transformers franchise, as well as Skeletor from the He-Man television series.
 * The head's appearance is very similar to that of the unmasked T-800 in the film The Terminator.
 * At dawn, Skeletron Prime gains a significant damage and defense boost. Due to the similarities between Skeletron and Dungeon Guardian, this "attack phase" was dubbed "Dungeon Guardian Prime" by players.
 * When killed, the upper head part looks like Skeletron's.
 * There is a chance any monster during Hard Mode will drop the Mechanical Skull if the player hasn't ever summoned Skeletron Prime prior to the 1.2 update.

Bugs

 * It is possible that when he dies, Skeletron Prime will vanish, not drop anything, and his limbs can still be attacked for a few seconds despite him being dead.
 * Rarely Skeletron Prime will not spawn when the player has used the Mechanical Skull or the icon The Air Is Growing Cold Around You has appeared
 * Sometimes at dawn, Skeletron Prime (Guardian form) will fly above you and make it easy to kill him with a sword, however you only do 1 damage.
 * Sometimes while wearing the Turtle armour at dawn he kills the player and himself (unconfirmed)