The Corruption

The Corruption is the purple half of the world evil coin, themed on rot and decay rather than the Crimson's gore. Purple grass, dark trees, Vile Mushrooms and thorny bushes that hurt on contact cover the surface, and the water runs purple. Its gear is generally quicker in the hand than the Crimson equivalent while hitting for slightly less, and its enemies are correspondingly a bit softer.

Corruption chasms drop in straight vertical lines, sometimes crossed by a horizontal connector partway down, and they go deep. Fall damage is the real hazard here, not Eater of Souls. Before your first trip, get something that handles the drop: a hook, a Lucky Horseshoe, a Featherfall Potion, or rope you lay as you descend. A hook covers both halves of the problem, because climbing back out is its own puzzle and a Devourer will happily knock you off a rope.

If you plan to live nearby, cover the chasm mouths with platforms. It costs a few minutes of wood and saves the fall you take running past one at night. Rows of platforms laid across the shaft at intervals also break the descent, which turns a chasm from a hazard into a staircase.

Every chasm ends in a Shadow Orb, with Demon Altars scattered through the passages around and inside it. Ebonstone walls the whole structure in and resists every early pickaxe until you have a Nightmare Pickaxe or better. Explosives blast through it, and Purification Powder turns it straight back into ordinary stone, which is usually the faster way in on a first visit.

The first orb broken in a world drops a Musket with ammunition, which satisfies the Arms Dealer move in requirement on the spot. The third one you break spawns Eater of Worlds where you are standing, so clear a lane before you swing at it.

Early on the biome pays out Demonite Ore and Shadow Scale from the worm, plus Deathweed and mushrooms for potions and powder. Rotten Chunk is the quiet one: it makes Leather, and Leather makes two thirds of the Archaeologist's set. In a Crimson world you cannot get it without a Corrupt Statue or a patch of Corruption you built yourself.

In Hardmode the underground half becomes the only source of Cursed Flame and one of two sources of Soul of Night. Corrupt Mimics down there carry the Dart Rifle, Chain Guillotines, Clinger Staff and Worm Hook, and every enemy in the biome has a thin chance at a Corruption Key for the Dungeon's biome chest.

In Hardmode the Corruption pushes into whatever it borders. A corrupted desert keeps the same music and the same core enemies and adds Dark Mummies on the surface, Dark Lamias, Vile Ghouls and Desert Spirits underground, and Bone Biters during a sandstorm. Sand becomes Ebonsand, and Waterleaf stops growing there, which matters if that desert was your Waterleaf supply.

In a snow biome only ice converts. Snow blocks are immune, so a corrupted snow region shows up as Purple Ice Block veins running through otherwise clean terrain. Purple ice can also exist from world generation where the two biomes happened to overlap, and it sits completely inert until Hardmode wakes it up.

A working Corruption biome needs 300 tiles. Below that you can still get Corruption enemies to spawn from as little as a single block, but fishing checks for the real biome, so crate farming needs the full count. Seeds, Unholy Water and Purple Solution all seed it, and after the Moon Lord you can shimmer Red Solution into Purple. What you cannot fake is the Steampunker's inventory: she only stocks the purple solution and the Decay Chamber if the world genuinely generated corrupt.

The design borrows openly. A spreading blight that twists the ground and warps whatever lives on it lines up closely with The Colour Out of Space, the 1927 Lovecraft story about a meteorite that poisons everything near where it lands, which is probably why meteors used to fall after you smashed an orb. The chasm shape reads as worm tunnels and answers to the worm at the bottom of them, while the Crimson's branching cavern reads as a neuron and answers to a brain.