Cults of the Dragon Below
Western Khorvaire was twisted long ago into a dark, dangerous land by the invasion of the daelkyr, the native inhabitants of the plane of Xoriat. These alien conquerors laid waste to cities and towns, and corrupted their inhabitants into foul abominations that still wander the deep places of the world. After centuries of the living nightmare of the daelkyr, the druids called the Gatekeepers managed sealed the planar portal and drove the daelkyr into the depths of Khyber using magic taught to them by the dragon Vvaraak. But the Dhakaani Empire had been shattered by their war against Xoriat, and the orcs and goblinoids degenerated into scattered clans (giving rise, eventually, to the modern state of Darguun).
The Shadow Marches are still scarred by that ancient war. Here more than anywhere else, remnants of the horrors the daelkyr unleashed yet lurk in the swamps and teh depths. Roughly half the orc inhabitants still follow a simple, rustic lifestyle and continue to observe the druidic traditions of the Gatekeepers and related faiths, but many others here in the remote western reaches maintain strange practices and observances. These sects are 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. Some worship daelkyr, while others worship the great Demon Overlords of myths and legends, and others still have found myriad semi-forgotten horrors to latch onto.
From this origin, the cults spread across Khorvaire and beyond. Any community that gives reverence to dark forces, particularly those 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.