Oracle of the Mournlands
The Dragon Below
Western Khorvaire was twisted long ago into a dark, dangerous land by the invasion of the daelkyr. The conquerors from Xoriat laid waste to cities and corrupted the inhabitants into foul abominations that still wander the deep places of the world. Using magic taught to them by the dragon Vvaraak, the Gatekeepers finally sealed the planar portal and drove the daelky into the depths of Khyber. But the Dhakaani Empire had been shattered by the war, and the orcs and goblinoids degenrated into scattered clans.
The Shadow Marches are still scarred by that war, and remnants of the horrors it unleashed yet lurk there. Roughly half the orc inhabitants still follow a simple, rustic lifestyle and continue to observe the druidic traditions of the Gatekeepers. Most of the others practice strange worship collectively known as the Cults of the Dragon below. Despite the all-inclusive name, these cults have nothing in common beyond devotion to the darkly insane forces within Khyber. They do not communicate with one another, and indeed they might have completely opposite aims.
From this origin, the cults spread across Khorvaire and beyond. Any community that gives reverence to dark forces, particularly thse purported to be buried deep in the earth, could be deemed to support a cult of the Dragon Below. Aside from their dark heart, Dragon Below worshipers are as varied as the forms of evil.