Underground Corruption

The Underground Corruption is the same as the Corruption, but it begins at the underground depth. It has even more dangerous enemies.

It is only accessible in the lowest of the Corruption chasms before the Wall of Flesh is killed, and until the Wall of Flesh is dispatched, it will be unable to spread. The Underground Corruption can only be made to stop spreading by surrounding it with Hallow or Underground Hallow biomes, which is notably difficult, as every trace of Corruption can spread more underground.

The Underground Corruption, as with the Underground Hallow, do not generate on new worlds, but rather when the world enters hardmode after the Wall of Flesh has been killed.

Unlike most of other biomes Underground Corruption does not suppress spawn of other underground and cavern enemies, but just adds own NPCs and Soul of Night drop to all.