The Creeping Nether is a mod which causes the Nether to spill into the overworld.
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.
Zombie pigmen can be purified into living pigmen.
Also the sky is red now.
Sandstone corrupts into soulstone, and soulstone purifies into hallowstone. Essence can be used to charge soulstone and hallowstone (crafting recipe in recipe book), amplifying their effects and allowing you to build catalysts.
Catalysts are crafted from dragon breath, charged soulstone/hallowstone, and essence. Recipe is in recipe book. After placing the catalyst, surround it with charged soulstone/hallowstone like in above screenshots for it to start having effect. It will take time for the radius to expand enough for efffects to be visible.
Stone and dirt will turn to Netherrack. Sand will turn to Soul Sand and grow Nether Wart. Cobblestone will be corrupted into a new block called Blood Stone. Trees will burn into charred wood, which drops charcoal.
Mobs from the Nether will also spawn naturally in the Creeping Nether.
The Creeping Nether will occasionally let loose a strike of lightning, transforming any mobs that are affected.
Villagers turn into witches, pigs turn into zombie pig men, and slimes turn into magma cubes.
Unable to handle this unstable energy, cows will explode and sheep will burst into flame.
Corruption will also occur in the Nether itself, but this can be turned off if you need.
Ores corrupted by the Creeping Nether will have slightly higher yields.