Skeletron

There is no summoning item the first time. Go to the Dungeon entrance after dark, talk to the Old Man standing outside, and pick the curse option. He becomes the boss on the spot. You get one attempt per night, because he will not come back until the next dawn, and killing him yourself achieves nothing except making you wait. This is one of only two bosses that needs a living NPC to exist at all, the other being the Wall of Flesh.

Once it is dead the curse lifts and the Old Man leaves for good. After that, the only way back to the fight is the Clothier Voodoo Doll: equip it and kill the Clothier at night, and the boss spawns on him. It is repeatable, which matters if you want more of its drops.

Walk into the Dungeon before beating it and Dungeon Guardians appear as soon as you drop past the surface. They hit for effectively unlimited damage and kill almost anyone in one touch, and beating one does not open the door either. It is technically possible to leave this boss until much later, since progression does not strictly require it until Golem is down and you need the cultists to appear, but skipping it costs you the whole Dungeon: no Mechanic, no Shadow Key and therefore no shadow chests, no Safe, no Void Vault.

It is a floating skull with two hands that move independently, and all three parts pass through blocks, so terrain does nothing for you. Only the head has to die. The fight alternates on a fixed cycle: a long stretch where the head hangs back and the hands take swipes at you, then a shorter burst where the head starts spinning and chases you directly. It roars every time the spin begins, which is all the warning you need.

The spin is the damage window, because its guard drops while it is spinning, but it is also the part that kills people. Avoid sitting directly above it or drifting far out to one side, because it accelerates to get back into position and the head and hands arriving together is very hard to read. A grappling hook is the most reliable escape from the chase, and a hopping mount does the same job.

Take both hands off before touching the head. On Classic that just removes the swipes. On Expert it is close to mandatory, because each surviving hand piles defense onto the skull, so early damage into the head is largely thrown away, and even the spin only drops its guard to nothing once both hands are gone. Moving fast enough on a minecart can make the hands give up and despawn, which is a shortcut if you have a long track ready.

The hands swing more often and faster, and they return to the head more quickly after each swipe. Contact with them applies Slow, and the head applies Bleeding, which shuts off your natural healing. Shimmering a Bezoar into an Adhesive Bandage blocks that bleed and makes every regeneration buff you brought actually work.

Once a hand dies or the head drops below three quarters health, it starts firing homing skulls, and the rate doubles once both hands are gone. Those skulls die against solid blocks, so a wall between you and the boss lets you plink at it with a yoyo in relative peace. The spin is also much faster, and it gets faster the further away you are, up to a large multiple at the far end of an arena. Staying close is safer than running, and running in a straight line is the classic way to get caught and pinned.

At dawn it enrages permanently. It stops behaving like a boss and starts behaving like a very fast Dungeon Guardian, with damage and defense set to numbers that kill anything it touches and cannot be dodged away with accessories. It stays like that until it dies or despawns, even into the following night. A kill in that state still counts and still drops everything, but almost nobody manages it. If the sky is lightening and the head is still healthy, leave and come back tomorrow.

The Dungeon opens permanently for everyone in the world and the guardians stop appearing. The Clothier can move in and the Dryad can too if she could not already. Items made with bones become reversible in Shimmer. Underworld enemies start rarely dropping the Cascade for as long as the world stays in pre-Hardmode, and Hell Slimes begin replacing the occasional Lava Slime. The Merchant adds safes to his stock, the Arms Dealer will sell you a Quad-Barrel Shotgun if you talk to him in a graveyard, and the Angler gains a couple of new quest fish. Old shaking chests also begin turning up in the Cavern layer, and opening one with a Golden Key in hand lets out an Elder Slime.

There is an alternate version dedicated to a Terraria streamer. Sit on Chippy's Couch at night with the voodoo doll equipped and the Clothier nearby and you get it instead. It hits harder, spins more often, moves faster in both phases, reflects your projectiles while spinning if either hand is still alive, and drops small groups of Dark Casters during the spin that teleport and fire faster than the usual ones. It always leaves behind its own clothing set on top of the normal loot, and it posts sponsor read jokes in chat while it spins.