Perhaps the forces of Chaos envied their joy, or ennui drove one of their number to rebellon or madness, perhaps jealousy or some rivalry drove them to summon uncontrollable powers, or some immutable Law decreeded that change must come to the unchanging. To the pitiful survivors, the cause is unknown and probably irrelevant. Their world ended when the planet was shattered in two; the land at the planet's habitable twilight dropped away, made into dust, or simply vanished. A whole disc was simply ripped out of the planet, which is now two hemispheres separated by a vast airy space, charged with swirling energy of Chaos and black aurorae of uncontrolled magical emanations.
Descendants of the servitors who weathered this apocalyptic moment eke out a living on the edges of this gap, on the chunks of material floating within, or on rugged vehicles they construct or salvage which traverses the interior void. They are few and far between; most of the planet has been given over to the horrific beings of Chaos, who scuttle and linger in the darkness of Ginnungagap. Some, though, have taken forms to inhabit the harsh twin half-worlds on either side of the void.
One of these half-worlds worlds is Muspellheim, the fire realm, bathed in fierce light and radiation by the three suns. The flat deserts are almost deserted of all but the seething wind-things, which are tasked by the multitude (and multiform) demon warlords of the rift valleys and jagged mountain caves with carrying messages and watching the borders of the realm.
The other is Niflheim, the rime-place, a black expanse shrouded in the swirling icy mists made by scouring winds blowing from Muspellheim. Unlike the ever-changing fire realm, Niflheim is in the grip of one single being, a multi-aspected, face-stealing demoness named Hel. The creatures which serve her claim that in her ice-spined castle, at the uttermost pole of Niflheim, she holds uncountable souls in crystalline stasis, and that all living are destined to become her prisoners upon death.
The inhabitants of Niflheim, Muspellheim and the Ginnungagap raid each other for prisoners, food, or what treasures remain from of the Gods' great lost civilisation.
This is just an idea I had this morning, along the lines of 'If the New Gods came after Thor, Odin, et cetera, what was the world before Asgard like?' And so, Ginnungagap by way of Jack Kirby. A later post will have some locations.