A deal's a deal: the cultists are sitting pretty up in the mountains, out of the way of the usual travelers. Head out to the broken bridge in the Burrwitch Outskirts. Once there, hug the cliff side until you find a path leading into the hills.
When you find men in blood-soaked robes beckoning the darker powers, you're in the right place. The entrance to their sanctuary is past the camp, carved into the mountainside.