The Folk of Osmon

From publisher blurb:

A mighty civilization once thrived where now only lonely Osmon Mire stretches across the land. The crumbled and vine-laden ruins of ages-old buildings arise here and there from the reedy mud and water. The remains of statues and derelict temples adorn low hills rising from the muck. Fell beasts roam the mire at night and man-like shapes haunt the swamp. After dark none willingly passes the low hill, with its blood-encrusted altar stone, where the Folk of Osmon are said to sacrifice their victims.