The Twins

The Twins are a Hard Mode boss similar to the Eye of Cthulhu, and the only source of Souls of Sight. The boss is made up of two conjoined enemies: Spazmatism and Retinazer.

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Overview
The boss fight can only be initiated by using a Mechanical Eye. Although the Mechanical Eye requires Souls of Light, that are only available in a world where the Wall of Flesh has been defeated, The Twins can technically be summoned in any world, regardless of whether or not Hard Mode has been unlocked in it. The Twins will only remain as long as it is Night, and will flee the battle at dawn.

Upon death, The Twins are guaranteed to drop 20-30 Souls of Sight and 5-15 Greater Healing Potions. Defeating them is the only way of obtaining Souls of Sight.

The Twins initially consist of two eyes connected together by a "rope" of flesh, one named Spazmatism and the other Retinazer. They maintain separate health bars, and are not considered defeated until both are killed (the first killed Twin will not leave any loot.) Each of these two eyes has a unique AI routine, as well as a second form that it will transform into when it is below half health.

Retinazer (First Form)
In its first form, Retinazer will shoot lasers at the player and occasionally close in slowly to make a series of melee attacks. The lasers do not pass through blocks.

Spazmatism (First Form)
Spazmatism, in its first form, will shoot cursed flame projectiles at the player and occasionally make a series of lunges at the player. The projectiles do not pass through blocks.

Retinazer (Second Form)
In this form, Retinazer gains significantly more defense and a much more frightening laser attack. It will always float above or to the left of the player and use its laser attack. When it is to the side of the player, it will begin to shoot its laser in rapid-fire bursts. It will never attempt to use its melee attack, preferring instead to maintain a constant distance between itself and the player. Note that in this form, Retinazer will not even attempt to shoot at the player if it does not have a clear line of sight, and its lasers do not pass through blocks. The lower Retinazer's health goes down, the faster the lasers become, making him even more dangerous before his death.

Spazmatism (Second Form)
Spazmatism's second form is the most threatening part of The Twins, as it possesses the most defense, the most damaging attacks and the greatest mobility. It will alternate between rapidly charging at the player to use its insanely powerful melee attack, and slowly giving chase while using its medium-range cursed flame breath attack. In this form, Spazmatism will not use the cursed flame attack if it does not have a clear line of sight. Also, Spazmatism's second form looks like the Eye of Cthulhu's second form.

General Tips
This section contains a list of quick tips that players can incorporate into their strategies.

Strategic Tips

 * You should try to avoid fighting both Twins in their second form at the same time. Try to kill them off one at a time. Since Spazmatism is statistically the deadliest of the two, you may wish to begin by eliminating him.
 * Caution is advised when fighting at high altitudes where the Wyvern can spawn.
 * Play the game on a high resolution or in full screen so that you can see The Twins from further away. This can help you to predict their movement patterns more easily.
 * Because Retinazer (Second Form) never attempts to use a melee attack and will not fire at you unless it has a clear line of sight, once Spazmatism is beaten you can build a small "box" with an opening and shoot at Retinazer with protection from most shots.
 * Take extreme caution when fighting both Retinazer and Spazmatism in their second forms; Spazmatism's cursed flame breath combined with Retinazer's laser attacks can kill you off fairly quickly. Using ranged means of attacks are recommended, as well as good armor.

Offensive Tips

 * Do not use melee or short ranged weapons against The Twins. They generally deal less damage than ranged alternatives, while simultaneously making yourself an easier target for The Twins' powerful melee attacks.
 * In order to deal damage quickly with as little risk to yourself as possible, use powerful long ranged weapons. The best weapons for this fight are the Crystal Storm, Megashark (if not availible to your player yet, consider using Clockwork Assault Rifle) and the Star Cannon. Note that Crystal Storm should always be used with the Mana Flower.
 * Make good use of all damage-boosting items available. This includes items such as the Sorcerer Emblem, Magic Power Potion, Bowl of Soup and Crystal Ball for casters. For ranged-weapon users, consider items such as the Ranger Emblem, Archery Potion and Crystal Bullets. Armor set bonuses can also increase your damage output, as can Reforging.
 * When the target you're shooting at is moving, always "lead" it by aiming slightly in front of its intended path. This will ensure that a greater percentage of your projectiles hit.

Defensive Tips

 * Always come to the fight with the best defensive equipment possible! Full Adamantite Armor is recommended for this battle. You should strongly consider the possibility of using the Adamantite Helmet for the increased defense, although you will not gain increased damage with your ranged weapons. Hallowed Armor is also useful for the fight with the Twins if it is available.
 * The Obsidian Shield and Cross Necklace are both useful accessories that can greatly boost your defensive capabilities.
 * It is recommended to Reforge some or all of your accessories to obtain defense boosts.
 * Don't forget about the wonders of the Ironskin Potion.

Movement Tips

 * Wide open areas allow for more freedom of movement. Enclosed areas will hinder your movement options.
 * Effective dodging requires both vertical and horizontal movement. A large number of items can allow for increased mobility, such as the Gravitation Potion, Angel Wings, Demon Wings, Rocket Boots, Wood Platforms, Magic Mirror and Grappling Hook, just to name a few.
 * Practice dodging ranged attacks by fighting against Corruptors or Gastropods. This may help you to improve your dodging skills before taking on The Twins.
 * You can instantly engage the speed boost of the Spectre Boots or Hermes Boots by using the grapple hook trick, which is extremely useful for running.
 * A good way to avoid attacks is to build a sky bridge, get Hermes Boots or the Spectre Boots and a Swiftness Potion, and run backwards while shooting either eye. The laser misses you most of the time and you can easily jump over the cursed fireballs.

Healing Tips

 * Items such as the Regeneration Potion and Band of Regeneration are not particularly helpful in this battle.
 * Always make use of any Heart Statues in your possession. They are one of the best healing methods in the game. Remember that each heart statue is worth 2 hearts every 8 seconds!
 * The Philosopher's Stone reduces healing potion cooldown by 15 seconds, which makes healing potions far more effective in boss battles.
 * If you set your spawn point to be inside the Nurse's room, you have a Magic Mirror, and you're fighting not too far away from your house, you can quickly teleport home in an emergency and immediately restore yourself to full health. Afterwards, remember to get away from your house immediately so that the boss doesn't kill your NPCs.
 * If you're fighting in an area of your world that the Hallow and Corruption have not spread to, weaker enemies will spawn there. Killing them is a free source of hearts--this is most effective during a Blood Moon.

Strategies
The following is a list of strategies that players can employ to defeat The Twins. Remember that making good use of certain tips above can greatly increase the effectiveness and success rate of these strategies.

1) Sky

 * This strategy hinges on the usage of the Gravitation Potion, possibly in conjunction with the Featherfall Potion to constantly fly around in the air, dodging the vast majority of attacks that The Twins unleash. It is recommended to equip an accessory for increased air mobility, such as the Rocket Boots and/or Angel Wings. It is also absolutely necessary to have an accessory that negates knockback, such as the Obsidian Shield or the Cobalt Shield.
 * When attacking, keep in mind that the bosses will constantly home in on you, so try to anticipate their movements and to not reverse gravity too suddenly, or they might move out of the way of your projectiles. Aiming can be simplified by luring the Twins to the top of the world and then falling straight down as they follow you, but beware of the Wyvern.

2) Skybridge

 * Simply build some Skybridges and run on them, using a Grappling Hook or other mobility-enhancing item to quickly switch between them to move out of the way of a melee charge or barrage.
 * This strategy can work wonders by building a Nurse's house at the start of the skybridge and setting the spawn point there. When low on health the player can teleport back, have the nurse heal them and resume the fight before the Twins have a chance to harm her. Beware that if the player has walked too far from the spawn point, teleporting will make the bosses despawn.
 * This strategy can also work quite well if a player uses a very fast weapon (such as the Megashark or the Clockwork Assault Rifle.)

3) Statue Corridor

 * For this strategy, employ plenty of Heart Statues. Using a wide wire grid to connect at least a row of them to pressure plates will cause them to drop plenty of hearts, allowing you to rapidly skip through them and regain lots of health. You can place a row of statues pretty much anywhere to take a small break from the fight in any moment.
 * Note that this strategy can be combined with a variant of Sand Coffin for maximum effectiveness.

4) Sand Coffin

 * This strategy takes advantage of the short invincibility the player is granted after taking any damage. It requires some Sand (or Silt, Pearlsand, Ebonsand, Spikes, etc.), a Cross Necklace, some healing potions, a good defense rating and a Cobalt Shield or Obsidian Shield to negate knockback. A Philosopher's Stone is also recommended.
 * To execute the strategy, simply stand in an open space from where you'll have a 360° line of sight, then have some sand fall on you. The sand will continuously deal 1 damage, thus negating most damage from other sources. Keep in mind that some attacks might still hit you, but that can be remedied by regularly consuming potions.

5) Hellstone Brick Skybridge

 * Even more effective than the Sand Coffin strategy and almost foolproof is the strategy to build a skybridge out of Hellstone Bricks. Without wearing an Obsidian Shield, Obsidian Skull or Obsidian Horseshoe, the player will take damage (only 1 in damage with good armor on) while standing on the skybridge fighting The Twins. This small damage and the invincibility that follows it will very effectively protect the player from the much higher damage that The Twins' attacks do.
 * Even though it's possible to defeat The Twins using this strategy without late-game items it is in any case recommendable to bring potions, an as good an armor as possible and keep a check on the health bar.
 * Wearing a Cross Necklace increases the length of invincibility from any damage taken, thus greatly improving using this strategy.
 * Beware of standing "inside" or very close to any of The Twins too long. Doing so greatly increases the risk of taking damage from The Twins instead of the hellstone bricks and this could kill even an experienced, well-equipped player fast. Jumping or flying is for the same reason not recommended during the battle, since you're not touching the Hellstone Bricks in air.
 * Around 250-300 Hellstone Bricks in length is recommended for players new to fighting The Twins because of the room for moving and avoiding The Twins' attacks it provides.
 * This strategy is just as effective for fighting Skeletron Prime and The Destroyer.
 * As long as you continue to take damage using this strategy, you will be able to avoid almost all other damage.
 * It's highly recommended to kill Spazmatism (the green eye) first, since it is the most dangerous.

Multiplayer Strategy
With the help of one or more friends, fighting the bosses becomes much simpler. With one player luring Retinazer away and taking cover to avoid its attacks, others can concentrate their fire on Spazmatism and kill it quickly, and successively go back to Retinazer and defeat it. Be careful, as if either the player luring Retinazer or all of the ones fighting Spazmatism are defeated, one of the bosses might despawn, making it possible to get the spoils from beating only one twin.