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 enemies joined by a "rope" of flesh: Spazmatism and Retinazer.

The Twins will sometimes spawn at night during Hard Mode, with the message, "This is going to be a terrible night... " appearing that dusk.



Overview
The boss fight can be initiated by using a Mechanical Eye, or by waiting for The Twins to spawn normally (although this might take a while). 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, similarly to the Eye of Cthulhu.

Upon death, The Twins are guaranteed to drop 20-40 Souls of Sight, 5-15 Greater Healing Potions, and Hallowed Bars. 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 (A trophy can be dropped from one twin while the other twin is still alive). 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 flames projectiles at the player and occasionally make a series of lunges at them. 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 named Death laser. 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 it even more dangerous before its 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 similar to the Eye of Cthulhu's second form.

Tips

 * Avoid damaging both Twins at the same time, as fighting both of them in their second form can be devastating. Focusing on Spazmatizm first is the safer choice, as it's more dangerous and should be eliminated as soon as possible. After it is beaten, the player can make a small box with some openings for their attacks to fight Retinazer, as it never charges at the player in its second form and will not shoot unless it has a clear line of sight.
 * Extra caution is needed for their second forms as they can deal devastating damage. Keep a good distance from Spazmatizm once it starts spinning and stay on the move to avoid Retinazer's lasers.
 * Base level, recommended equipment for this fight is Adamantite Armor or Titanium Armor.
 * Recommended accessories for this boss are the Star Cloak, Philosopher's Stone, Cross Necklace, and Wings or their upgrades to increase survivability.
 * Melee is not recommended for this boss, as most melee weapons available at this stage are lacking. If the player has defeated Destroyer beforehand, a full stack of Light Discs can be used.
 * The best choice of weapons for this boss are the Megashark(after Destroyer), Clockwork Assault Rifle and Crystal Storm.
 * The player should bring buff potions/items relevant to their weapon/class of choice. Such as the Archery Potion(for archers), Ammo Box and Ammo Reservation Potion for gunners, Magic Power Potion, and the Mana Regeneration Potion and Crystal Ball for mages. Generals buff potions are Ironskin Potion, Regeneration Potion, Endurance Potion. The player should place Heart Lanterns, Campfires and Heart Statues around their arena will also help.
 * The player should begin the fight promptly at 7:30 pm to give themselves more time to fight the boss.

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/Demon Wings. It is also absolutely necessary to have an accessory that negates knockback, such as the Obsidian Shield or the Cobalt Shield.
 * When attacking, the player should keep in mind that the bosses will constantly home in on them, so try to anticipate their movements and to not reverse gravity too suddenly, or they might move out of the way of their projectiles. Aiming can be simplified by luring the Twins to the top of the world and then falling straight down as they follow the player, 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 the 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 the player to rapidly skip through them and regain lots of health. The player can place a row of statues pretty much anywhere to take a small break from the fight in any moment.

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 impossible to get the spoils from beating one twin.



Name Origins
Both of the bosses' names are portmanteaus, and somehow related to the eyes: Retinazer is made of Retina and Laser, and Spazmatism of Spasm and Astigmatism.