This campaign is set on a floating island (at least initially.) I'm using Spelljammer-esque physiques since anything resembling real science leads to a lot of asphyxiated humans and demi-humans and the Undead laughing their heads off.
The island was part of a larger world called Aarde. It is roughly crescent shaped. I haven't decided if the mother world exists still or there are only islands floating in the mist.
There is no gravity as we understand it. Rather elements seeks their natural level. Levels from top to bottom are fire, air, water and earth. If you take these substances they will naturally fall into layers. As a result of this atmospheres are sharply defined. Flying more than a few thousand feet above the ground you encounter vacuum. The good news is you carry a small bubble of air with you.
The Island is fairly sunny and warm. As one approaches the edge the temperature drops till you encounter a rim of snowfields, frost and glaciers. That's essential to keeping the waters of the small sea mostly contained. As it is the sea and many rivers bleed off the edges. Beyond the Island they freeze and circle about or fall back in and add the ice flows that eventually move to warmer regions and melt.
The humans call their land The Wise Frost realizing it keeps their lands from drying out.
I'm not sure if the Island broke off the edge of Aarde or had a more central location. The original position depends on how much of Aarde I decide is intact.
The disaster happened at least two centuries before. That's enough time for the creatures of the Island to evolve or mutate (it's a magical world after all.) Unlike our world Aarde evolution follows Lamarckism. Attributes an organism acquires are passed to its offspring. Organisms living near the edge quickly develop the characteristics of arctic animals with odd D&D twists: ice dragons ice elves, ice worms etc.
Lamarckism has some interesting implications for role play. If your character is a fighter and survives combat after combat he will evolve attributes to help him in future combats. In other words leveling up becomes not just a matter of experience but evolution. You increase in HP because your body is evolving to need them. This explains why adventuring types level up while running around out in the wolds as opposed to practicing their swordplay and spellcraft in schools. Evolution baby!
It also means you pass on some of this to your children. Great fighters tends to sire great fighters or at least kids with that potential. Ditto for spell throwers. Paladdins are rare because they (usually) take vows of chastity. On the rare occasion they do have kids, think Sir Galahad.