Skeletron Prime

Use a Mechanical Skull after dark and it arrives immediately. It can also turn up by itself: once you are in Hardmode and have smashed an altar, there is a small chance each dusk that it announces itself with a line about the air turning colder, provided you have never beaten it in that world. The share is small at first and improves as you kill the other two mechanical bosses, since the three of them split the same roll.

A natural arrival is easy to lose by accident. The actual spawn happens well over an hour of game time after the message, and it is canceled if you are below the surface at that moment, if another boss is already up, if a Lantern Night is running, or in single player if you leave the world in between. Summoning it by hand ignores all of that and cancels a Lantern Night outright. In a world combining the For the Worthy and Remix seeds you cannot summon the mechanical bosses individually at all.

The head does not chase you outside one specific window. It parks itself above you and slightly to one side, correcting hard whenever you move out from under it, and lets the limbs do the work. Only the head has to die.

Each arm has one job and a preferred position, and reading the position is most of the dodging. The cannon sits overhead and lobs bombs upward rather than at you, which means they come down on you afterward. The saw stays below and grinds up and down. The vice hangs on a diagonal, flailing and occasionally lunging in. The laser holds a horizontal line and fires aimed shots. The bombs do not break terrain but they do explode against platforms and planter boxes, so a platform floor gives you no cover from them.

Every ten seconds or so the head grows spikes and charges you the way its pre-Hardmode ancestor does, for a few seconds at a time, with a roar to announce it. Every limb escalates at the same moment: the cannon aims properly and fires far more often, the saw homes in on you directly, the vice lunges more, and the laser roughly doubles its rate.

The important difference from the original is that its defense doubles alongside its damage while spinning, rather than dropping. Emptying your best cooldowns into the head during that stretch is close to wasted output. Treat the spin as pure evasion, get vertical, and go back to attacking when it stops. On Expert the charge is much faster and speeds up further the further away you are, so distance is not the answer it feels like.

Rushing the head is possible and usually worse. Each limb you remove deletes an entire attack pattern, and once the head is on its own the fight is simple. Start with the laser, since its shots are the hardest to see coming and it sits along the line you naturally travel. After that, take whichever one is disrupting you most rather than working around in order.

It carries a lot of defense, so Ichor is worth applying even though the weapons that inflict it hit softly. Put the debuff on, then switch back to your real weapon. The head resists Penetrated but the limbs do not, so a piercing debuff still has a target.

Free movement matters more here than raw defense, because the spin doubles what contact costs you. Wings cover the vertical, and a long flat run or a sky bridge covers the horizontal. Keep the bridge solid rather than platforms so the cannon's bombs land somewhere other than under your feet. Running too far is its own failure: get past roughly 375 tiles away and it dives and despawns, ending the attempt with nothing.

If it lives to sunrise it enrages permanently. The limbs stop mattering, it charges at absurd speed, and contact kills outright, with no help from dodge accessories or a shield. A kill in that state still counts and still drops loot, but in practice a build that cannot finish it inside one night needs better gear. Break the skull early in the night, not halfway through it.