The Twins

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

The Twins are the Hardmode version of the Eye of Cthulhu. They can only be summoned at night with the Mechanical Eye and must be defeated before sunrise, otherwise they disappear. Similar to the Eye of Cthulhu, they have a chance of spawning when night falls once Hardmode is enabled, displaying the message "This is going to be a terrible night..." at dusk.

Both eyes have their own individual life meter and both must be killed in order for the Twins to be defeated. The eye with the red iris, Retinazer, fires purple eye lasers while hovering above and to one side of the player. The one with the green iris, Spazmatism, shoots balls made of Cursed Flames, while hovering parallel to the player. Both alternate between their signature attacks and a charging attack, much like Eye of Cthulhu. Note that Retinazer's charge dash is faster and covers a bit more distance than Spazmatism's charge dash.

Just like the Eye of Cthulhu, they will both transform when their health drops to half. Retinazer becomes a mechanical eye with a laser turret in its pupil, while Spazmatism becomes a mechanical mouth. Notice that Spazmatism's second form looks like the Eye of Cthulhu in his second form. Retinazer will start firing lasers more quickly and for more damage, and gains the ability to shoot a quick barrage of lasers. It alternates between harrassing the player with periodic fire and slowing down for rapid-fire. The more damaged Retinazer is, the faster it will shoot lasers during its rapid-fire stage. Spazmatism will begin actively chasing the players and constantly spewing Cursed Flames from its mouth, in a manner similar to the Flamethrower; the flames can inflict the Cursed Inferno debuff upon players and deal high damage. It alternates its flamethrower attack with a much more aggressive charge/dash that has more speed and range.

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 rope connecting the twins is similar to the rope connecting The Hungry to the Wall of Flesh.