Dungeon Guardian

Nothing summons it. Walk into the dungeon in a world where Skeletron is still alive, drop below the line where the surface becomes underground, and one or more of these spawn and come straight for you through the terrain. To count as inside the dungeon the game wants a lot of dungeon brick around you and unsafe dungeon wall behind you, which is why the entrance hall and the outside of the building are safe and the first descent is not.

While it can spawn, it takes over everything else. No bound mechanic, no angry bones, nothing else in the dungeon appears, so there is no sneaking past to loot early. It does obey the normal spawn cap, so more than one can arrive at a time when the slots are free.

One touch ends the run for almost any character, and the damage ignores the dodge effects that would normally save you, so hallowed armor and ninja gear buy you nothing. It travels through blocks, so there is no wall to put between you. It is faster than most wings. Debuffs do not stick to it. Everything that usually gives you an out has been removed on purpose: the intended lesson is that the dungeon is closed until you beat its boss.

Its armor is set so high that every weapon in the game, at every stage of a run, scrapes off a single point of damage per hit and double that on a critical. That is the entire design. There is no gear check to pass and no build that changes it.

You have a couple of seconds, so keep a teleport ready: a Cell Phone, a Magic Mirror or a Recall Potion all work if you trigger them before it reaches you. Landing short does not help, since it will follow until you are far enough away for it to despawn. A Depth Meter is close to required if you plan to explore the upper dungeon at all, because the trigger is a depth line you cannot see. And to be clear about the name: killing one does not open the dungeon. Skeletron is still the only key.

People do it, and the reward is a cosmetic pet, so this is a trophy hunt rather than a step in progression. Because damage per hit is capped, the only stats that matter are how many hits you land per second and how often they crit. That points at the fastest repeating weapons in the game rather than the strongest: the Zenith, the Phantasm, the Chain Gun or Megashark with an endless ammo source, the Piranha Gun for not needing to aim, and minions like the Blade Staff, Terraprisma or Stardust Dragon Staff chipping away in the background. Almost everything on that list comes from after Skeletron, which means the usual approach is to take a finished character into a fresh world.

The other half is staying ahead of it. Ordinary wings will not do it. A Witch's Broom or a fast mount will, as will hovering wings paired with the Soaring Insignia, and a Rod of Discord helps at the world edge where you run out of room. If you would rather remove the dodging entirely, a circular hoik loop with a boomerang, a line of Teleporters, or a Portal Gun all keep it permanently unable to touch you. Do the whole thing in a Classic world: Expert and Master add nothing except more health, and the fight is already measured in thousands of hits.

A drunk world pushes the trigger depth further down, so you can meet ordinary dungeon enemies without setting one off. Combine drunk with celebration and they do not spawn at all, which lets you walk into the dungeon before Skeletron and take everything in it. That is the one legitimate exception to the wall.

Originally the dungeon was patrolled by weak copies of Skeletron's head, and clearing them out let you in without fighting him. This was built to close that hole, and it kept the old sprite. It was meant to be unkillable, but it was never actually made invulnerable, and in 2011 a player uploaded a video of himself killing one. The developers responded by adding the pet drop so the effort would at least pay something. It still has a map icon despite not being a boss, and on Master it holds more health than any other enemy outside the events.