The Twins

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The Twins are a Hardmode mechanical boss which is essentially a more difficult version of the Eye of Cthulhu. The Twins are two separate flying entities, Retinazer and Spazmatism, connected by a tendril, each with its own attack pattern and life count. Both must be killed in order to defeat the boss. Retinazer is usually the one which is defeated last because it is usually harder to hit thanks to its lasers.

The Twins can be summoned manually using a Mechanical Eye at night (7:30PM to 4:30AM). They also have a 10% chance to spawn randomly at dusk once a Demon or Crimson Altar has been destroyed, as long as The Twins have not yet been defeated at least once in the current world, with their arrival announced by the status message "".

The Twins must be killed before dawn, otherwise they fly up and then despawn.

Eyes
There are two eyes: Retinazer, and Spazmatism. Retinazer is a laser-shooting eye with a red iris. Spazmatism is a similar-textured eye, though it has a green iris. It fires green Cursed Flames.

First phase

 * The eyeball with the red iris, Retinazer, fires high velocity purple lasers. It will usually attempt to remain diagonally above the player.
 * The eyeball with the green iris, Spazmatism, fires medium velocity balls of Cursed Flame. It will usually attempt to remain horizontally in line with the player.

Both alternate between their signature ranged attacks and charging melee attack, much like the Eye of Cthulhu. Retinazer's charge attack has higher velocity but has longer periods between charges and does less charges than Spazmatism's.

Second phase
Each eye has a second form, which each individually reaches upon its health dipping to 40%. Retinazer transforms at /  /  HP while Spazmatism transforms at  /  /  HP.
 * Retinazer becomes a mechanical eye with a laser cannon in its pupil, and only fires projectiles, ceasing its attempts to ram the player (or align with the player diagonally). It fires more rapidly and with higher damage, and periodically fires in quick barrages which does significantly less damage. The barrages become more intense and frequent the more damage Retinazer takes, eventually occurring non-stop.
 * Retinazer will, however, align horizontally with the player when firing barrages, though not as tightly as first-phase Spazmatism does.
 * Spazmatism becomes a mechanical mouth (similar to the Eye of Cthulhu) that chases the player, charging with greater speed and range, and periodically spews streams of Cursed Flames from its mouth in a manner similar to the Flamethrower. The flames can inflict the Cursed Inferno debuff and deal high damage, so it is a good idea to use healing potions in this part of the fight. It no longer attempts to align horizontally with the player.

In Expert Mode, Retinazer's laser firing becomes faster and charges cover more distance as its health depletes. Spazmatism's fireballs inflict Cursed Inferno for /  the duration. Spazmatism also charges faster in its second form.

Tips

 * Focus on defeating one eye completely first without doing too much damage to the other one. Battling The Twins while both are in their second form makes the battle much more difficult, especially in.
 * Spazmatism may be a better choice for defeating first, since its cursed flames are difficult to avoid without proper mobility equipment/buffs in its second form. Retinazer is less deadly, as it cannot inflict debuffs and will not charge in its second form, which helps the player get some distance if they are a ranger.
 * A Cross Necklace helps during The Twins' second form, as players will take far less damage during barrage attacks.
 * The Twins are only immune to Confused, so using a weapon that inflicts a debuff may be useful during the fight.
 * A good arena for this boss is simply a long, flat plane. With Lightning Boots (or better) and a ranged weapon, the player can continually outrun The Twins' attacks while firing back at them.
 * The is an excellent weapon against The Twins if it is fired as the player runs away.
 * A with Holy Arrows can be a great tactic, as it can deal massive amounts of damage while staying out of range of many of The Twins' attacks.
 * The can be great for melee players, as it does not require any ammo or mana, and can hit both eyes if aimed correctly.
 * Retinazer's second form cannot attack the player if they are hiding in a box. Defeating Spazmatism then causing Retinazer to reach its second form can become an almost guaranteed win if this strategy is used.
 * A Gravitation Potion can be an excellent method of keeping distance from the Twins, allowing them to be attacked freely.
 * Farming for the Black Spot on Master mode could help bypass making an arena and make dodging/keeping distance a lot easier during the fight, though care must be taken not to accidentally despawn the Twins by moving away too fast.

Trivia

 * Retinazer's name is a of ', a part of the eye, and '. Spazmatism's name is a portmanteau of ', meaning to twitch uncontrollably, and ', an eye disorder.
 * Spazmatism's second form has visible bolts along either side, whereas Retinazer’s second form only has them along one side.
 * The Twins seem to have complete (two different eye colors).
 * In its second form, Spazmatism's fire stream attack functions almost identically to the Flamethrower weapon, with similar range and damage, but inflicting the Cursed Inferno debuff instead of On Fire!.
 * The Twins are the only boss with two trophies, and the only with phases.
 * Despite inflicting it, Spazmatism is not immune to Cursed Inferno.
 * The Twins are one of three bosses with a corresponding minion (Optic Staff), the others being the  and Duke Fishron (Tempest Staff).
 * In, The Black Spot mount travels faster than The Twins, allowing the player to run away and fire backwards for the entirety of the fight.
 * The Twins seem to be bio-mechanical hybrids since they are not immune to debuffs like On Fire!, Poisoned or Venom, even when transformed.
 * The achievement "Ophthalomologist" has two 2nd phase Eye of Cthulhus, despite getting this achievement from defeating The Twins.
 * The achievement "Ophthalomologist" has two 2nd phase Eye of Cthulhus, despite getting this achievement from defeating The Twins.