The Twins

The Twins are a hardmode version of Eye of Cthulhu boss. They can be summoned with Mechanical Eye or just at a Blood Moon.

Both eyes have their own individual life meter and the ability to teleport around. The red one, Retinazer, fires purple eye lasers while the green one, Spazmatism, shoots a flamethrower like stream that appears to be formed of Cursed Flames and makes short dashes towards the nearest player.

Just like the Eye of Cthulhu, when they have been hit for half of their health, they change. Retinazer becomes a mechanical eye with a laser turret in its pupil, while Spazmatism becomes a mechanical mouth. After changing, Retinazer will start firing lasers more aggressively, and his lasers will be red death lasers that do more damage, while Spazmatism will begin chasing the players faster and constantly spewing Cursed Flames from his mouth.

Trivia
Retinazer's name is a portmanteau of retina, a part of the eye, and lazer. Spazmatism's name is a portmanteau of astigmatism, an eye disorder, and spazm, meaning to twitch uncontrollably.