This setting was originally conceived of to be played in a Dungeons and Dragons 3.5e campaign, which eventually transitioned to “D&D Next”, which became D&D 5e. The original concept was to have a two-continent system, which involved a “new world” style exploration, where a new continent had been discovered across the seas, one full of monsters, wonder, and grand secrets. Much of my initial worldbuilding was put towards the first continent, the one where the majority of the traditional fantasy races would hail from, with intermixed political weavings and dealings helping define their goals both at home, and in this new world to be explored.
Through various happenstance and shenanigans, the new world exploration never came to fruition, as the party became wrapped up in plots within the old world itself. This eventually lead to adapting elements of the “Elemental Evil” campaign module, culminating in a grand final battle that ripped open the veil between the material world and the various planes beyond. This begat a planar merge, that shattered the world, smashed it together with the inner and outer planes, and created a post-apocalyptic style fantasy setting, complete with a magical “radiation” style system that the party would have to navigate!