It is the year 2999 AC. Rhea is a nearly-flat plate, floating in orbit around a blue-tinted star. Titanic forces swirl at the edges creating an impenetrable, turbulent storm system that only the most fool-hardy of sailors or mountaineers attempt to penetrate. Those who do are never seen again, though occasional wreckage does wash up on shores near the edge.
Way, way back, in a mostly forgotten history, Rhea was a thriving planet with almost five thousand years of civilizations rising and falling upon it. Until three thousand years ago, when a great cataclysm occurred, tearing Rhea apart. Known as The Shattering, life was irreversibly disrupted, and nearly extinguished. Of the surviving portion of Rhea, only a flat disc remained habitable. Life picked up the pieces, and began again.