Rund is a geologically unstable archipelago planet, dotted with thousands of irregular islands across its entire surface. The largest island is only a few hundred kilometres from tip to tip, and most are much smaller. Each is a tiny enclave in a vast stormy ocean that spans from pole to pole. There are tremendous climatic variations according to latitude: there are hundreds of arctic, temperate, and desert islands, with many subtle variations between them. Violent geologies abound throughout the globe; new islands are created and old ones destroyed every year by the rapidly shifting tectonic plates that crumble and grind against each other beneath the raging waves. The seas are shockingly deep, violent, and stormy, and even though many islands are within sight of each other, traversing the channels between them is extremely hazardous.
The oceans are also filled with a variety of dangerous creatures. From the deeps come armoured predators larger than Imperator Titans, and tentacled horrors driven from the pelagic depths by seismic upheavals. Vast shoals of vicious lyers leap from the sea into the air alongside stinging jellyish the size of halo barges, while clamberfish drag themselves aboard ships and pull sailors limb from limb. Worse are the repulsively intelligent Karkinaxi, the terrifying Sea Devils of Rundeen lore that are capable of cunningly luring men to their deaths through a variety of ruses. All these and more ill the planet’s endless waters. "Indeed, the world is so replete with such a rich selection of hostile creatures that some have speculated that they have all been brought here for some reason; perhaps the world was once a kind of pleasure world for big game hunters during humanity’s Dark Age of Technology."
Askelline Adeptus Administratum adepts still fervently debate whether Rund should be classified as a Death World or Feral World. Rund appears to have been inhabited by humans for thousands of years. No clear evidence remains of how human settlers first arrived, but given that the majority of the native Rundeen share "a certain commonality of physical characteristics, it is speculated that they are the descendants of some minor colony".