The Twins

. The Twins are the Hardmode version of the Eye of Cthulhu. They can be summoned at night with the Mechanical Eye and must be defeated before sunrise, otherwise they disappear.

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 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. 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

 * The Twins are the only hardmode boss with a second form.
 * 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 will disappear if the player(s) are too far away.
 * Unlike the Eye of Cthulhu, the twins gain defense when they take on their second form.
 * If the eyes are far enough from each other, the rope connecting them won't be visible until they get close enough again.
 * Occasionally, if the player moves fast enough after summoning them, only one of the twins will be in the fight. This is due to the boss being summoned, but then the player moves too far away from just one of them. Most often it is Spazmatism that survives and must be defeated.
 * Occasionally, when all players are dead in multiplayer, the player may respawn just after one of the eyes runs away. The other eye will return to the fight and the eye that ran away will be counted as defeated for the rest of the fight.
 * If Summoned at high altitudes, the twins may not appear; if they spawn from above where the map ends they disappear (all monsters that can pass through blocks, excluding Wyverns, that exit the map disappear).
 * In multiplayer, if more than one set of The Twins is summoned using server commands, Spazmatism/Retinaizer will be connected to the same or different eye of the opposite pair.
 * The Twins may be a reference to Skids and Mudflap from 2009's Transformers 2, as the characters were often referred to together as "the twins" and were respectively lime green and inferno orange.
 * The Eye of Cthulu, Spazmatism, and Retinazer all have the same sprite with different colored irises.