Worldwound

A demon-haunted wasteland that lies at the northern-most reaches of central Avistan, the Worldwound is an unnatural blight: it is a rift to the Abyss opened on the Material Plane. It appeared shortly after the death of the god Aroden at the beginning of the Age of Lost Omens, and is probably the greatest threat to life on the face of Golarion

Crusades
Hearing of the Abyssal incursions in the north, the church of Iomedae dedicated itself to closing the rift and stopping the demons' advance. Still reeling from the death of their goddess' patron, the god Aroden, the clergy decided that a crusade would unite the still fledgling faith and complete the task Aroden had begun against the followers of Deskari so long ago.

Since the First Mendevian Crusade was launched in 4622 AR, thousands of devout Iomedaeans have traveled across the Lake of Mists and Veils and up the West Sellen River to the Mendevian capital of Nerosyan to do battle with the demons of the Worldwound. Though they have beaten back the Abyssal onslaught numerous times, they have never been able to close the rift between the planes.

This is the beginning of the Fifth Crusade.

Link
http://www.pathfinderwiki.com/wiki/Worldwound