Long ago, before the Incursion, the subregions that make up the Severed Rimelands (Nurau, Raunu, and Aruun) were one whole region, under a name that has now been lost to history. At the start of the incursion, the River Styx cut through the land and split it into three separate parts.
The Incursion of fiends caused drastic alterations to the previously ideal land filled with open plains, forests, and mortal cities. Nurau was effectively razed by demons, Raunu turned into an arctic hell by the Archdevil Mephistopheles, and Aruun completely transmuted into black ice by an unknown force, and its inhabitants frozen along with it.
After the Gap, those in the demon-razed lands of Nurau (which was a vast majority of the still living mortals) awoke to a wasteland coated completely in an odd ash that was not warm, rather it seemed to inflict something close to frostbite when someone was exposed for too long. Many mortal cities had crumbled without an explanation, along with their residents, other than the "mortal beacon" city of Myddraal and a scant few others that fight for survival in the wasteland of ash.
To make matters worse, the other side of the styx that surrounds all of the regions is where all mortals who lived before the Gap (or otherwise learned it) know, for a fact, that three other regions were. Now, the other side of the styx is covered with an impossibly tall, burning wall of hellfire that separates the Severed Rimelands from any of these other regions: no mortal has been able to pass this wall thus far, not even those not affected by flames, as hellfire makes no exceptions for who it burns.
Raunu stayed a similar, snowy landscape, but any cities there before the Gap had now been completely reduced to rubble and ruins. Devils are known to torment any mortals that come into the land, usually resulting in those mortals disappearance... though it has been confirmed by magical detection that those disappeared mortals usually survive far longer than their disappearance, many still living now, but their whereabouts unknown. Though it is a primarily devil-dominated region, there are reports of a single human-sized demon near that roams near the ever burning Hellfire Pyre, that kills any and all of those that come near, devil, demon, or mortal.
Even before the Gap, Aruun was essentially barren after its transmutation into black ice. Almost no mortals ever go to the region, not even to mention the fact that in order to get there you would first have to pass through devil-scarred Raunu. Though, the thunderously loud cracking of ice can be heard every now and then even on the other side of the styx on Raunu, making those that hear it question if there is more to be seen beneath the endless ice.