The Destroyer

A Mechanical Worm calls it, and only after dark. It will also come on its own: once you are in Hardmode and have smashed at least one altar, every dusk rolls for a mechanical boss, and a hit prints a line about vibrations from far below. That is a warning, not an arrival. The boss itself surfaces well over an hour of game time later, and several things can cancel it in between. Being below the surface layer when the timer runs out kills the spawn, so does leaving the world in singleplayer, so does having any other boss alive at that moment. An Eye of Cthulhu rolling on the same night takes priority, and a Lantern Night blocks natural spawns entirely, though summoning manually cancels the Lantern Night instead of being blocked by it.

The odds are shared. All three mechanical bosses draw from the same dusk roll, so each one is uncommon while all three are alive and the chance concentrates as you kill them. The last one standing turns up quickly by comparison.

Dawn ends the fight whether you were winning or not. It burrows off and is gone, and a summoned attempt costs you the item. It never gives up on its own otherwise: no amount of running makes it lose interest, and it will follow you across the entire map. The only other way out is dying.

It comes out of the ground coiled and unwinds as it comes. On screen it needs something to tunnel through and handles badly in open air, which is the whole basis of a good arena: thin platforms strung over empty space give it nothing to swim in and keep it committed to long, readable passes. Off screen that restriction disappears and it moves freely, so it can be directly beneath you again the moment you stop looking. Watching the minimap for where the body is heading is more useful than watching your character.

The body is one health pool, so there is no shortening it by killing segments, and every segment is armed. It leaps out of the ground to ram you, then tries to wrap its length around wherever you are standing, and while it does that the entire body is firing lasers at you. Getting encircled is how most attempts end, because the pass that surrounds you closes off the direction you were about to move in.

Damage to the body shakes flying drones loose, and the light on a segment goes dark once it has given up its one. That makes the supply finite, and a long fight eventually runs it dry, but the ones already out are a genuine second fight. They never rejoin the body, they do not lose interest when you run, and killing the boss does not kill them, so a careless victory leaves you standing in a swarm that is still shooting.

Kill them as they appear. They drop hearts frequently enough that clearing them is closer to resupply than to a distraction, and a dark segment still fires lasers, so there is no reward for leaving them alone.

Anything that pierces beats raw single target damage here by an enormous margin, because one shot lined up along the body connects again and again on the way down it. The head has no armor at all, which makes standing in front of it the highest damage position in the fight and also the fastest way to die. Skip flasks and any weapon that leans on debuffs, since none of them stick to it. A gravitation potion lets you hover just above the body and out of contact range.

In Expert and above, the golden shower loses a chunk of its damage against both the boss and its drones, which is worth knowing if that was your plan for the mech bosses generally.

If you get boxed in, a mirror or a teleporter pad breaks contact without ending the fight, since it will simply come to you again.

Killing it does not close anything off. Another worm brings it back any night you like, which is how most people farm the souls it leaves. Its death also raises the nightly odds of whichever mechanical bosses are still alive, so the second and third fights come looking for you sooner than the first did. If you built the world with both the harder seeds turned on, none of this applies, because the three of them arrive fused together as one summon instead.

For scale, it is the longest thing in the game and holds more health in a single bar than any boss except the Moon Lord, whose total is spread across separate parts. The harder seed makes it longer still, with bigger segments. The lore has it as a mechanical copy of Cthulhu's spine, which is also why it looks nothing like the worm it is modeled on.