The Creeping Nether
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.
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.