The Twins

"This is going to be a terrible night..."

The Twins are a Hardmode Mechanical Boss which is essentially a harder version of the Eye of Cthulhu. The Twins are two separate flying entities connected by a tendril, each with its own attack pattern and life count (Spazmatism having the most life and damaging effects). Both must be killed in order to defeat the boss.

The Twins are summoned using the Mechanical Eye at night, and must be defeated before dawn, otherwise they despawn. If an Altar has been destroyed, they also have a 10% chance of spawning each night until the boss is defeated. Their approach is signaled at dusk via the chat message, "This is going to be a terrible night..."

While The Twins are alive, music Boss 2 will play.

Attacks

 * Retinazer: has a scarlet iris and fires purple lasers. It will usually attempt to remain diagonally above the player.
 * Spazmatism: has a lime iris and fires balls of Cursed Flame. It will usually attempt to remain horizontally in line with the player.

Both alternate between their signature attacks and a charging melee attack, much like the Eye of Cthulhu. Retinazer's charge attack is faster and covers a bit more distance than Spazmatism's charge attack.

Each eye has a second form. Retinazer transforms at 8000 HP while Spazmatism transforms at 9000 HP.
 * Retinazer becomes a mechanical eye with a laser turret in its pupil. It fires more rapidly and with higher damage, and periodically fires in quick barrages, but doing significantly less damage. The barrages become more intense and frequent the more damage Retinazer takes, eventually occurring nonstop. In its second form, Retinazer will only shoot at the player(s) and no longer attempt to ram into them.
 * Spazmatism becomes a mechanical mouth. It 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.
 * In Expert mode, Retinazer's laser firing becomes faster and charges become farther as its health depletes. Spazmatism's fireballs inflict Cursed Inferno for a longer length of time; sixteen seconds instead of eight. Spazmatism will also charge faster in its second form.

Tips

 * Both of the Twins are vulnerable to the Frostburn, Ichor, and Cursed Inferno debuffs. As such, a viable combination is Frost Armor, a Megashark (if The Destroyer has been defeated), and either Ichor Bullets or Cursed Bullets. The Twins will be inflicted with two debuffs simultaneously.
 * In both of the Twins second form, using items like a Cross Necklace or having Titanium armor greatly helps take less damage by becoming invincible during most of their strong "Rapid Attacks"
 * If the Twins are ever out-run, such as with the case of a long railroad, they will despawn regardless of the time of night. This allows you to only have to fight one of the two Twins, and will most likely be Spazmatism due to it always trying to follow you.
 * Try to defeat one of the two Twins at a time. Having both in their second forms at near equal health will prove dangerous to any player, especially in Expert mode.
 * Try to beat Spazmatism first since its cursed flames are difficult to avoid in its second form and fighting both of them at the same time makes the fight significantly more difficult. As well as this, Retinazer is less deadly, cannot inflict a debuff and will not charge in its second form. It is also easier to outrun Retinazer and therefore get away from it quicker whilst shooting a Megashark or a Hardmode Repeater from Hardmode Ores.
 * Because Souls of Sight are used mainly for making magic based items, this boss is less rewarding to players going for ranged or melee/warrior builds.
 * Shadowflame Knives and Cursed Flame-based weapons are an excellent against The Twins, as neither of them are immune to the Shadowflame and Cursed Inferno debuffs.
 * The is an excellent weapon against The Twins if it is fired as the player runs away.

Trivia

 * Retinazer's name is a portmanteau of retina, a part of the eye, and laser. Spazmatism's name is a portmanteau of astigmatism, an eye disorder, and spasm, meaning to twitch uncontrollably.
 * The Twins are the only Mechanical Boss with phases, and is also the only mechanical boss that seems to be partially made of flesh, possibly being biomechanical hybrid. Unlike the other two, they are vulnerable to more debuffs.
 * Despite inflicting it, Spazmatism is not immune to Cursed Flames.
 * They are the only boss with two trophies.
 * Although The Twins spawn off the screen, the tendril between them can spawn in the screen.
 * The Twins are one of three Bosses with a minion designed after it, the others being the Martian Saucer, the Xeno Staff, Duke Fishron, and the Tempest Staff.
 * The achievement appears to depict the second form of the Eye of Cthulhu rather than The Twins.

History
Die Zwillinge Les Jumeaux 쌍둥이 双子魔眼