Approximately 100 years ago The Worldwound opened in northern Avistan. The demons did not immediately pour forth though, but, showing remarkable restraint for their kind, trickled in and did not immediately cause the lands surrounding the Worldwound alarm. It was only after the demons completely destroyed the kingdom of Sarkoris that the neighboring kingdoms realized the danger, but by then it was too late.
Commonly, "Worldwound" refers to the area conquered by the demons who spread out from the actual Worldwound, which is a half mile long rift to the abyssal plane. Sages and wizards argue about what caused the rift, but no one knows for sure. What is known is that once the rift opened demons from the abyss began entering Golarion by the hundreds and more pour forth every day.