Evil biomes

A new world rolls the Corruption or the Crimson at even odds, or you pick one at creation. Only special seeds hand you both. That single roll decides more than the color of the ground: whether you break Shadow Orbs or Crimson Hearts, which altars you smash, which biome chest waits in the Dungeon, which underground evil forms when Hardmode begins, which biome mimic a Key of Night produces, and which conversion solution the Steampunker is willing to sell you.

The two are near mirrors with a tilt. Corruption gear tends to swing faster while Crimson gear hits harder, and Crimson enemies carry a little more health, defense and damage to match. The bigger difference is terrain. Corruption chasms are straight vertical shafts, so fall damage is the main thing that kills you down there. Crimson chasms open from an angled entrance into a wide round cavern with narrow branches trailing off it, so falls are rarely fatal but every fight happens in a corridor with nowhere to back up.

Both biomes hide the same progression trigger at the bottom of their chasms. The first one you break in a world hands you a gun and a stack of ammo, which is enough on its own to bring the Arms Dealer to town: a Musket in Corruption, The Undertaker in Crimson. Three broken in total summons the biome boss, Eater of Worlds or Brain of Cthulhu, immediately and wherever you happen to be standing. Break the third somewhere you can actually fight.

The rest of the pool is a small one time weapon set: Vilethorn, Ball O' Hurt and Band of Starpower on the Corruption side, Crimson Rod, The Rotted Fork and Panic Necklace on the Crimson side. Most of it gets replaced quickly, but the mana band and the necklace both hold an accessory slot for a while.

You are not locked out of the opposite evil forever. Housing the Dryad in a Graveyard during Hardmode makes her stock the other biome's grass seeds. A drunk world generates both. After the Moon Lord dies you can shimmer Purple Solution into Red Solution and back. Failing all that, you carry blocks over from a second world.

There is a catch worth knowing before you invest the time. An artificial biome counts for enemy spawns, fishing and key drops, so farming the opposite Biome Key there works fine. It does not place the matching biome chest in your Dungeon, and it does not change what the Steampunker stocks. Her solution and her matching block generator are tied to what the world generated with, not to what you built.

Killing the Wall of Flesh creates Underground Corruption or Underground Crimson in the Cavern layer, with a different enemy list and a different soundtrack from the surface version, and not necessarily beneath the original biome. That layer is the only source of Soul of Night, which anything killed there can drop. It also holds the biome specific material: Cursed Flame from Corruption enemies, Ichor from Crimson ones, both of which feed weapons you will use for the next several bosses. Every enemy in either evil biome carries a 1/2500 chance of a biome key.

Both evils convert nearby tiles, barely before Hardmode and aggressively after. The standard answer is a trench: three tiles of open air stops block to block conversion, but thorny bushes near the surface reach further, so surface trenches want closer to six tiles or a lining of non-corruptible blocks. Beating Plantera once cuts the rate in half.

For actual removal you want the Clentaminator and Green Solution from the Steampunker, who arrives after your first mechanical boss. Before then, Purification Powder handles small patches and Sunflowers hold a line. Do not sterilize the world once you can, though. The evil biome is a supply you keep drawing on, and the Hallow will expand into whatever space you clear.

Where the evil overlaps a desert, sand becomes Ebonsand or Crimsand and the area gains desert specific enemies with their own drops. In a snow biome only ice converts while snow blocks stay clean, which is why snow resists better than anything except mud. Jungle grass converts too, and that one genuinely hurts: an infected Jungle stops growing what you go there for. Green Solution turns it back, which is the main reason to buy a Clentaminator at all.