The Crimson

The Crimson is the flesh themed world evil, sitting opposite the Corruption's purple rot. Red grass and trees, Vicious Mushrooms, thorny bushes that damage you on touch, red water. Its weapons and armor land a bit harder than the Corruption equivalents while swinging slightly slower, and its enemies carry more health, defense and damage to match.

A Crimson chasm begins as an angled tunnel down from the surface, widens into a large round cavern, then sends narrow branches outward and downward from that cavern. Each branch dead ends in a Crimson Heart. Compared with the Corruption's vertical shafts you take far less fall damage, but the trade is that every fight happens in a tight tunnel where Face Monsters and Crimera reach you from angles you cannot see coming. Bring something with reach, or something that bounces.

Crimstone seals the chasms and needs a Nightmare Pickaxe or Deathbringer Pickaxe to mine, though explosives and Purification Powder both open a path without one. Crimson Altars line the passages on the way down.

The first heart broken in a world drops The Undertaker with ammunition, which brings the Arms Dealer to town. The third summons Brain of Cthulhu on the spot, so break that one somewhere with floor space rather than at the end of a finger tunnel where you cannot dodge.

The heart pool covers roughly one class each: The Rotted Fork for summoners, Crimson Rod for magic, Panic Necklace for anyone who keeps getting hit. The boss pays in Crimtane Ore and Tissue Sample, and in Expert its bag holds the Brain of Confusion.

Hardmode moves the good material underground. Ichor drops only from Crimson enemies, and the debuff it inflicts strips enemy defense, which is why ichor ammunition and flasks stay in rotation for the rest of the game regardless of class. Crimson Mimics guard Fetid Baghnakhs, the Dart Pistol, Life Drain, Flesh Knuckles and the Tendon Hook. Soul of Night drops from anything killed down there, and every kill in the biome has a small chance at a Crimson Key.

In Hardmode the Crimson eats its neighbors. A crimson desert plays the same music with the same enemies plus Blood Mummies above ground, Dark Lamias, Tainted Ghouls and Desert Spirits below, and Flesh Reavers during a sandstorm. Sand becomes Crimsand and Waterleaf will not grow. Where it meets snow, only ice gives way, leaving Red Ice Block threaded through untouched snow. Both overlaps can also exist from world generation, sitting dormant until the Wall of Flesh falls.

300 Crimson tiles make a real biome. A world generating with Corruption does not stop you building Crimson inside it, nor stop that Crimson spreading afterward. Going the other direction, buying Corrupt Seeds from the Dryad while she is housed in a Graveyard during Hardmode gets you a Corruption patch without touching a second world. The one thing you cannot fake is the Steampunker's stock: she sells Red Solution and the Flesh Cloning Vat only in a world that generated Crimson.

The chasm layout is doing real work. A round chamber with threads branching off it is a neuron, which is exactly the boss that lives inside it, while Corruption chasms are worm burrows and answer to a worm. The entrance even carries stalactites arranged like teeth. Crimstone being the flesh of something enormous and unseen fits the rest: the crafting station you use to work it is called the Meat Grinder.