Wall of Flesh

It never appears on its own. You have to kill the Guide in lava, in the Underworld, while he is alive to begin with. Throwing a Guide Voodoo Doll into the lava does both at once, and letting him burn to death down there works just as well no matter what actually killed him, as long as he was touching lava at the time. The boss then forms well outside your screen on whichever side that happened and starts moving toward you. That single detail sets the direction you will be running for the whole fight, and because of how the spawn position is chosen it can end up on the side of the world you were not expecting. The game also stops it short of the true edge of the map, so triggering the fight while standing near a world border still puts the boss behind you rather than in front.

The Guide is the bottleneck for repeat attempts. He cannot move back in at night or during an event, so a failed try costs you a long wait. Finishing a fight just before dawn is the standard trick for squeezing two runs out of one evening, especially if his house is already built. Check he is actually back before spending another doll.

Lay a flat run of at least a thousand blocks across the Underworld before you summon anything at all. The choice between solid blocks and platforms is a real trade rather than a preference.

• Solid bridges cut off the lower eye's line of fire and let bouncing weapons like Beenades work, but they also swallow your own shots and remove the option of dropping through to dodge.
• Platform bridges let everything travel in both directions and give you a fast vertical escape, at the cost of leaving the bottom eye free to shoot.
• On Expert and above, build platforms out of stone, ash wood or obsidian. Dying Lava Slimes will destroy wooden ones under your feet mid fight.
• If you would rather skip the construction, Lava Waders or a Water Walking Potion let you run on the lava instead.

Only the two eyes and the mouth take damage, and all three feed one shared health pool, so it makes no difference which of the three you attack except that the eyes are softer. Everything else on the body is just scenery. The eyes also shift up and down along the body to slide past blocks and liquid near them, which means the surface you built partly decides where the lower eye ends up firing from.

There are no real phases, just steady escalation. It fills the screen top to bottom and sweeps in one direction the entire time, so you are always retreating and always dealing with the Underworld's usual hazards. As its health falls it moves faster, and by the end it is genuinely quick. The eye lasers gain damage, speed and rate on the same curve, so the last stretch is a wall of purple arriving while you run out of bridge.

The two things it sends after you are The Hungry, the small mouths hanging from its body on veins, and Leeches spat from the mouth. Kill The Hungry early. They gain damage and toughness as the main body takes damage, and once one has absorbed enough it tears off its vein and flies at you far faster than the wall moves. Both they and the Leeches leave hearts, which on a bare bridge is usually the only healing you have.

One of them stops you leaving the fight. While it is on you, touching the body itself deals a fixed hit with none of the usual random variance and shoves you the way the boss is facing, normally into more of the same. That knockback goes through knockback immunity, so a Cobalt Shield or anything built on it buys you nothing.

The other one is the tongue, which reels you back toward the boss if you outpace it. It also ignores knockback immunity. In Journey Mode with godmode on, teleporting out does not kill you the way it normally would; you simply get dragged all the way back down instead.

• The Hungry only sometimes leave a heart instead of always, but they respawn for as long as the boss lives, so swinging at them never stops paying.
• In multiplayer they gain knockback resistance per extra player and stop flinching entirely with a full group.
• The tongue's pull scales with the boss's own speed, which stops it from simply outrunning you at low health.
• The treasure bag holds a Demon Heart for a permanent extra accessory slot. Only the first one you eat counts, and later ones stop appearing.

Its loot is left inside a small floating frame of brick where the mouth was, which is also the only time in the game a boss places blocks for you. The Pwnhammer is the piece you cannot skip: it is the first hammer strong enough to break altars, and breaking altars is what puts Hardmode ore in the ground. The Merchant and the Angler both pick up new stock and new requests afterward.

Then the world changes for good. Two diagonal bands, one Hallow and one of your world evil, cut from the sky down through the Underworld in a V, and both spread far faster than before. Your jungle and your NPC housing are on a clock now. You can refight it as often as you can feed the Guide to the lava, but the conversion only happens once.

Summon it by accident and you can leave the world and rejoin to make it vanish, though that also deletes anything lying on the ground and returns you to your spawn. If every player in the Underworld dies, the screen fades to black over a few seconds and the boss leaves by itself. On a For the Worthy world the eyes and mouth shrink while The Hungry grow into the main threat, and reinforced Fire Imps keep spawning throughout. One last quirk: it does not roar when summoned, so nothing warns you but the status message.