Not quite the end. You see, to maintain their constructions, the dwarves use sourced water and magma to manufacture obsidian out of infinite resources, or in some regions may just make glass blocks to build with. As population increases, so does the need for rooms, and the need for walls. However, the slow process of generating matter eventually reaches a point where it can no longer be sustained, and the mass of dwarven achievement begins to become... problematic. As the dwarves mine and build and mine and build, farming obsidian and making glass objects, it all leads towards one inevitable event: at some point in this distant future, when the mass of the world has been impossibly expanded, some miner digs where he oughtn't have dug...
A section of the cavern has collapsed!
At this point, the doom of the world cannot be stopped. All dwarven creation collapses in on itself. The depths, where hell used to be, are crushed into a furious, glowing core that radiates violet light, while much of the former "Underground", with less pressure above it, is simply crushed to a skin Slade, most dense of all stones, above the core of violet light. above that, the heat of the collapse melts many stones and leaves others semi-molten, while layers of rubble accumulate on the surface.
Some life escapes destruction, trapped in air pockets upon the Slade or in the rubble, with very few upon the surface, reduced to utter barbarism. The violet light of the core transforms those close to it. The closest survivors become multitudes of foul creatures, while those few, poor souls left in the new underground become unique entities, as do some on the surface in regions where the radiation invisibly seeps from below.
There come cool rains from the clouds of dust that enshrouded the world when all its land was reshaped, carving new rivers in new mountains. The spores of above-ground plants, drifting through the atmosphere, find purchase as the shattered stone is worn down into soil, while the underworld is enriched as water seeps down below. Forests grow, and the radiation of the core dies down from its initial fury, leaving no new great monsters to be created upon the surface, where the dwarves, elves, goblins, and kobolds scurry about in their shadows. Deep beneath, upon the surface of the Slade, the trapped survivors worship the light that wells up from pits that collapsed where the Slade was thin and multiply with its power until they are as numberless as they are hideous.
The world has survived, but it is fundamentally changed: new mountains exist where chance left the highest constructed spires in taller heaps, sedimentary layers formed where the rains of the first dark years washed the particles needed for their creation. Volcanoes burn and lava cools into igneous stones, while the collapse itself formed metamorphics from whatever was there to fall. And amidst it all, titans and night creatures roam, perhaps massive bronze statutes wrought by dwarves past in their arrogance walk animate, or birds become bloated to titanic size. Some of the mutants even eerily resemble the dragons of old...
The dust clouds part and the sky clears. The peoples of the world stand up once again, and declare that day when a glorious new dawn breaks through the murk to be the beginning of a new age, the very dawn of time.
Welcome to Year 1, the Age of Myth, when living gods and mighty beasts hold sway.