The planet is tidally locked. Most of the thriving species have settled in the middle ground, which they affectionately call The Belt. The Great Dark lies away from the sun, a place that rapidly descends into freezing temperatures and eventually massive glaciers, strewn with frozen over mining networks. and the remains of five ancient cities.
Long ago, noble folk struck a deal with devils to bring blazing sunlight to The Great Dark for 1,000 years. From them, incredible innovations of magic and tinkering sprung forth, while the lineage of nobles was slowly tainted by the blood of devils to give birth to what are now called Tieflings. The greatest of them were known as the Red Kings.
But they do not rule anymore. At the end of the 1,000 year bargain, the brilliant sunlight went dark, and so did the warmth they brought to the great cities built beneath them. Many fled for their lives into the harsh winter, some getting lost, many making it to The Belt, and a few... are still trapped to this day.
Towards the scorching desert lie few settlements, some of them little more than a collection of shanties built around shallow oasis. But a few bastions of civility perch in sand and the flame. It is said a city of brass was erected in a place where sand becomes glass, and another lies shrouded behind wavering sheets of heat like a veil of illusion. This place is treacherous and deadly in it's own right. This is Sunward.