Wrath of the Righteous
Orcs surfaced from beneath Avistan before the arrival of Aroden on Golarion, having been displaced by dwarves as they ascended to the surface. During this time, Golarion was affected by a cosmic event that kept the planet under darkness for thousands of years.
Orcs prospered during this time, as there was no direct sunlight to affect their vision, and during this time they established small townships throughout Avistan and Garund. Though they have retained maintained a firm grasp in the Hold of Belkzen in central Avistan, but have lost much of their influence in the rest of the world.