Eye of Cthulhu

You can call it deliberately with a Suspicious Looking Eye, used after dusk. Only one can be summoned at a time, and using the item during the day or while one is already alive does nothing at all, it is not consumed and no message appears.

It also arrives on its own, rolling for a spawn on any night until you have beaten it once. The world checks three things: that somebody has built up to roughly two hundred maximum health and more than ten defense, that four townsfolk are living in the world, and that you have not already killed it. The Old Man, the Traveling Merchant and town pets all count toward the four. The Skeleton Merchant does not.

When the roll passes you get a message about an evil presence watching you, and the eye itself does not appear until well over an hour of game time later. That gap is your preparation window. It is also your escape route: if you spend the rest of the night below the surface layer it never spawns, and coming back up before dawn brings it straight back. The cheaper trick is to head it off entirely by taking off an accessory or two before dusk, since dropping under the defense threshold means the roll never happens.

It moves through terrain the whole fight, so an arena of walls buys you nothing and a flat run with platforms is worth far more. Early on it hangs directly above you, drips out Servants a few at a time, then commits to three dashes back to back before starting the cycle again. The Servants read as pure harassment, but they leave hearts and stars where they die, and that is most of your sustain for the fight. Killing them as they spawn is usually better than ignoring them.

Around halfway down it stops dead and spins in place while it sheds the shell. That pause is the only genuinely free damage window the fight offers, and it is short, so have something big loaded before you reach it. What comes out the other side stops spawning Servants entirely, has no defense left at all, hits considerably harder on contact, and roars every time it charges. It rams three times in a row with much more commitment, then hangs in the air for a moment. That hang is your other window.

It gives up under three conditions: dawn arrives, every player dies, or it drifts far enough off screen. Outlasting it is a legitimate option on a bad attempt, and running until the sun comes up costs you nothing but the summon item.

More Servants throughout, plus a batch dumped out the moment it transforms. Those are survivable. The chain dash is the problem: in the second phase it gains a rapid string of charges, announced by a higher pitched roar, which starts out as an extra on top of the normal dashes, then replaces them, then speeds up as its health falls until the last stretch is close to continuous charging.

The compensation is that its defense goes negative near death, twice over, so every hit you land in that window lands harder than the same hit did at full health. Trading a little safety for damage output at the end genuinely shortens the worst part of the fight rather than just feeling brave.

The summon item is trivial to make and the fight is short, which makes repeat kills one of the better pre-Hardmode income sources. Crimtane Ore sells for noticeably more than Demonite does, so the same effort pays better in a Crimson world.

Its ore alone covers a sword, a bow, an axe, a yoyo, a whip and a fishing pole. Only the armor and a few tools also want Shadow Scales or Tissue Samples, which it does not drop, so two or three kills fund most of an early weapon set without touching the world evil boss. There are no location requirements on the summon either, which means you can kill a few of them inside a boss arena you are about to use for something harder and leave the hearts on the ground as a mid-fight supply.

It stops spawning naturally, and the Dryad becomes willing to move in if she was not already. The Angler opens up quests for Angelfish and Cloudfish, and the Traveling Merchant adds Code 1 and the Gray Zapinator to his rotation. If your world somehow has no altars left standing, it drops one on death, which is the game quietly making sure you cannot lock yourself out of Hardmode.

The summon item keeps working forever, so it is refightable as often as you like.

• It is named for Lovecraft's Cthulhu, as are several other bosses. The lore has it as an organ torn out of him during an old war.
• It was one of the three bosses in the game at launch, alongside the Eater of Worlds and Skeletron.
• The Merchant starts muttering about an evil presence once your world meets the spawn conditions, which makes him a free early warning system.
• Its trophy is one of its teeth, which only exist in the second form. Kill it fast enough with late game gear and you can finish it without ever seeing them.
• It has central heterochromia, two colors in the one iris.
• It sometimes drifts through the night sky in the background, and two cloud shapes based on its forms can appear up there as well.
• It has guest appearances in Dungeon Defenders 2, Palworld, and Don't Starve Together, where it goes by Eye of Terror.