As previously written, there is a world, the surface of which is consumed by Hellfire, unleashed perhaps by an Avatar of an ancient fire-lord, intent on ascension... but alas, it was all for naught, or maybe not, but regardless, the world that the Avatar of Fire, Lord of the Last Flesh, sought to arise to dominate, was instead dominated by raw fiery power and consumed.
Consumed that is, save for a few isolated motes of humanity that struggle on. As also previously explored, some of those motes of humanity exist in carved out spaces deep within the bowels of the earth, existing in dependence upon harnessed hellfire, known to the dwellers below as sun-shards... those communities are linked via long tunnels which are traveled by some equivalent of a subway, more conventional train cars or carriages, or perhaps akin to the pneumatic tube cars or floating subways envisioned in the 19th century. Into that complex of subterranean city states, there is also a surface access leading to a mountain top fortress with a standing gate, locked down and attuned to a surviving satellite-city (the hellfire spread through unsecured portals and consumed many of the satellite cities).
That forms one confederacy of human endeavor toward survival, but there are others...
A large circular valley, formed by an ancient blast, is know as the God-Wall, and its diameter is in excess of a hundred miles. Within is a large teardrop lake its point reaching the mountainous wall of the crater at the site of an ancient construction: a flight of steps rise from the lake, each standing at least as tall as a man, rising up the slope some distance, terminating at a large semi-circular landing, before a huge upright circular construction made of a mysterious mineral, appearing to be a closed portal of some sort. Spotted throughout the valley, chiefly on its perimeter, are small structures of similar ancient design, the insides of which are adorned with a multitude of glyphs, the meaning of which is lost to antiquity. The upper slopes of this valley are covered with woods filled with savage creatures, forcing the human denizens to live close to the lake. Other points of interest: Several locations where trails exist that lead to mountainous overlooks, giving both dramatic views of the valley, as well as the Hellfire beyond the God-Wall. An ancient breech in the mountains, that was sealed with a wall of similar build to the ancient structures within. Peculiar ruins deep under the surface of the lake. The main centers of human existence are on the lake, on fixed islands, or even some free floating islands made of reeds.