Crimson

Crimson is a biome, introduced in 1.2. This biome can randomly replace Corruption in a world when it is created. It is similar to corruption in manner it is generated: stone is replaced with crimson stone, there is crimson grass and there are deep chasms, which contain big pulsing hearts, which replace Shadow orbs. When smashed by a hammer, they work like Shadow orbs with the exception that they summon Cthulhu's brain instead of Eater of Worlds.