Blackwood Swamp surrounds the Taldan port of Cassomir on all landward sides. It is a murky, rotting, brackish marshland of tangled trees, dense overgrowth, quicksand, man-eating plants, menacing hydras, marauding humanoids, and other monsters. Once it was part of the primeval forest that covered the region. Cassomir was long ago cut from this swamp and the city constructed an enormous moat to protect the city from the monsters. The road that travels through the Blackwood Swamp from the main gates of Cassomir is an elevated causeway constructed of wooden beams and thick stone piles. It is always patrolled by the Taldan Phalanx, and most merchant wagon trains travel the swamp accompanied by guards.
The Pathfinder Society has long claimed that the Blackwood Swamp is located atop the ruins of an ancient Azlanti city. No one has found any ruins within the swamp, but the murky land and the dense vegetation could conceal anything, even ancient ruins.