When you build a nether portal, it will begin to slowly corrupt the land until all that is left is a scorched hellscape. There is a limit to how far the corruption can spread however, and the corruption can be halted by breaking the portal.
New in version 2.2:
More configuration options to control what blocks will corrupt into. Default configuration supports Biomes O' Plenty.
Nether essence can be obtained by mining nether ores in the overworld. Nether essence can be used to corrupt blocks much like a portal.
This essence can be purified by washing it with a ghast tear. Once purified, it can be used to purify nether blocks into "Hallow" blocks.
Netherlight roses will sometimes grow upon hallow grass. They are a flower that emits light.