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 can be spread through artificial means.
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.
The Crimson 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 is a biome which can wield the dungeons 'spider cave' and spawn a variety of blood 'crawling' monsters.
Post-Hardmode, the Crimson gets several new monsters, which are:
- The Crimson in the map looks like a human hand (or hands)
- 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.
- You cannot cleanse the Crimson with The Corruption or The Hallow.
- When asked about the status of the World, the Dryad refers to the Crimson as the Corruption.
- The Crimson may look like an anatomic heart when in a cave.
- All Ores replaced by The Crimson are better than in worlds with The Corruption eg. Titanium replacing Adamantite in Crimson worlds yet it makes better equipment.
- 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.
- Crimson Altars seem to frequently appear out of The Crimson. This may be a bug.
- Added to the game.