Retinazer

A Mechanical Eye used after dark calls them straight away. They can also show up uninvited: in Hardmode, once an altar has been broken, every dusk rolls a chance for one of the three mechanical bosses, and you get a warning line about the night ahead. That warning is not the arrival. The pair actually forms some time later that evening, so being below the surface at the moment it happens, or leaving the world during the gap, means nothing turns up.

Plenty of things cancel a natural spawn. An Eye of Cthulhu arriving takes priority, any active boss or the Old One's Army blocks it, and a Lantern Night stops it too, though using a summon item overrides the Lantern Night the way every boss summon does. The odds also concentrate as you clear the set, so the last mechanical boss standing is much more likely to turn up on its own than the first was.

Retinazer and Spazmatism are separate creatures joined by a tendril, each with its own health and its own behavior, and both have to die. Killing only one pays you nothing at all, not even the hearts a boss normally leaves. Dawn ends the attempt: if they are still alive they flee. In single player they can only leave together, so a fight going badly cannot be rescued by letting one slip away. Multiplayer lets them despawn separately, which a group can exploit by timing deaths so one leaves while somebody respawns, leaving a single eye to mop up.

The red eye, Retinazer, hangs at an angle above you and fires fast shots with genuinely poor aim, and it moves more slowly than its sibling. A player who is standing still is a surprisingly bad target for it at this stage. The green eye, Spazmatism, tries to hold level with you and lobs balls of cursed flame that usually leave you burning. Both mix ranged attacks with charges the way the Eye of Cthulhu does, but on different rhythms: the red one commits to a few long runs with real gaps between them, while the green one strings together a rapid burst of short lunges.

Each eye changes on its own once its health falls past its own threshold. It spins, sheds the retina, and hangs inert for a couple of seconds before the new form starts working. Both become much harder to damage from that point on.

• Retinazer stops charging entirely and turns into a turret. Its accuracy goes from terrible to exact, and it alternates a steady stream of heavy shots with weaker rapid barrages, both speeding up as its health drains until the end is close to continuous. It lines up horizontally with you for the barrages and hovers above you otherwise.
• Spazmatism becomes a mouth that simply pursues you, alternating a run of long fast charges with several seconds of a sustained flame stream that reapplies the burn for as long as it touches you.

The usual first-attempt mistake is spreading damage evenly and flipping both into their second forms at the same time. That version of the fight is far worse than either half alone, and it is much worse still in Expert. Spazmatism is the one to remove: its burn keeps ticking while you are busy dodging the other eye, and its second form gives you nothing to hide behind. Retinazer is survivable by comparison. It never charges again once it changes, so a reasonably tanky build can stand its ground, keep damage up, and only move to break line of sight.

Nothing elaborate is required. A long flat run with Lightning Boots or better lets you outpace both patterns while firing backward, because you never have to stop and turn into them. A Gravitation Potion opens the vertical space instead if you prefer holding distance that way. Transformed Retinazer needs a clear line to you and has no alternative attack, so sealing yourself inside a box takes it out of the fight completely while you heal or reapply buffs. That does nothing at all against Spazmatism, which is one more argument for the kill order. The pair shrugs off only Confused and Poisoned, so any other debuff weapon you own is worth bringing.

Both eyes fire faster and charge further as their health drops in the first phase, and their projectiles travel noticeably quicker, so the early section stops being the free damage window it is on normal. After the change, Retinazer's barrage rate scales much harder against its own missing health and even its slower shots move faster, while Spazmatism trims the pause between individual charges down to almost nothing.

Their death pushes you toward Plantera, since bulbs only start growing once all three mechanical bosses are gone, and it raises the odds of the remaining two showing up on their own. They are the only boss that hands out two separate trophies, and both can land in a single fight. The names are eye jokes: Retinazer folds retina into laser, which is exactly what it becomes, and Spazmatism runs spasm into astigmatism, which fits an eye that twitches across the arena and blurs what it touches. Lore puts them among the mechanical rebuilds of Cthulhu's missing organs, though whoever built them left them fleshy enough to still catch fire.