The Crimson

 is a biome which was added in the 1.2 update. The biome is an alternate version of The Corruption, and has a 50% chance to replace the Corruption in newly created worlds. (Only one of the two can exist and fully form at the start of a new world.) Thus, it is functionally the same, with The Crimson spreading further in Hard Mode, corrupting Snow and Desert biomes as well. It has several new enemies, Pulsating Hearts that resemble Shadow Orbs, and Crimson Altars which resemble Demon Altars. The Underground Crimson will not be created after defeating the Wall of Flesh if the world was originally created with Corruption, but it can be spread through artificial means.

The Crimson can be spread artificially by placing Crimstone Blocks, Red Ice Blocks or Crimsand Blocks near a corruptible blocks, e.g. Stone Blocks, Ice Blocks. The Clentaminator can also be used to spread The Crimson. 500 blocks of Crimson-related blocks are needed to create an artificial Crimson biome. More blocks could be required if there is a large amount of blocks related to another biome nearby.

The Crimson will spread into the Surface and Underground Jungle once Hard Mode is activated. See Underground Jungle for more information.

The red appearance of the biome, the flesh-resembling tree leaves, the amount of insects and many gore-themed items obtained by Crimson materials suggests the biome's theme is based heavily on decaying flesh, instead of shadow-themed Corruption.

When 3 pulsating hearts are broken, the third heart will spawn the Brain of Cthulhu.

biome does not have chasms like The Corruption does. Instead, it contains a more web-like structure; a large, circular open area that supports 4-5 branches leading to the hearts which can be considered as the core for The Crimson biome. The caves contain Crimson Altars in place of Demon Altars, however hitting them with anything other than the Pwnhammer will still harm the player.

The shape of these Crimson structures looks very similar to a human heart when viewed on the map.

The Crimson may or may not be based on the movie War of the Worlds, as the Crimson is populated by creatures, blocks and foliage appearing to be made of organs or, like in War of the Worlds, blood.

Monsters
350px|right is a biome which can wield the dungeons 'spider cave' and spawn a variety of blood 'crawling' monsters.


 * Blood Crawler
 * Face Monster
 * Crimera

The Brain of Cthulhu can only be spawned in The Crimson by means of breaking three Pulsating Hearts or by the use of a Bloody Spine within The Crimson.

Post-Hardmode, the Crimson gets several new monsters, which are: All the post-hardmode monsters in The Crimson have a chance of dropping a Crimson Key Mold, though this is extremely rare (1:2500 prior to 1.2.3). The Crimson Key Mold can be obtained before the defeat of Plantera.
 * Floaty Gross
 * Ichor Sticker
 * Crimslime
 * Crimson Axe
 * Blood Feeder
 * Blood Jelly
 * Herpling

Weapons and Tools

 * Blood Butcherer
 * Tendon Bow
 * Deathbringer Pickaxe
 * Vampire Knives
 * The Rotted Fork
 * Crimson Rod
 * The Undertaker
 * Blood Lust Cluster
 * The Meatball
 * Flesh Grinder

Trivia

 * The Crimson looks like a heart or a human hand on the map.
 * The Crimson was first shown in the image shown to the right, which is a Steam background image of the biome drawn by Jimmarn.
 * Pulsating Hearts on the minimap (expandable fullscreen map) are labelled Shadow Orb when hovered (no longer true as of 1.2.3).
 * The Corruption or The Hallow will not spread into The Crimson.
 * You will know when creating a world that it will have the Crimson instead of The Corruption if it says "Making the world bloody" instead of "Making the world evil" during creation.
 * If one looks closely, one of the Crimson backgrounds features what appears to be a mountain sized skull and some bones blending into the scenery. Another features prominent stone archways resembling vertebrae.
 * Since the biome is based on necrotic tissue, the circular covered entrances to the Crimson chasms could represent "disease"-filled (aka enemy-filled) boils in the "flesh" of the biome itself.
 * The Crimson is like a living biome that eats the world.TerCrimston.png
 * Introducing or infecting an NPC village or similar biome may cause reoccurring Blood Moons.
 * The Meat Grinder is only dropped in Hard Mode.