An artificial moon that circles the elemental planes as a means of balancing its weather patterns. It was created to harness the powers of the planes, a "Lens", for magical experiments, but the wizard lost control of his toy when his artificial life form, the Shardmind of this universe, grew too intelligent and learned to harness the power of the lens from inside the planet.

The planet is fairly hollow, with a crystaline core where the Shardmind come from that gathers the power of the nearby planes. Shardmind themselves cannot touch the light of the planes directly, or they'll risk dissolving into that element.

Various humans and other creatures have started to inhabit the outer shell, unaware of what the planet's history is or what lurks underneath. Seasons are harsh, elementals are common, everything down to the dirt is downright magical.

I'm having the overarching plot start with Drow trying to steal supplies, wizards, and other important things in the middle of the night, as they prepare to defend against their greatest foe yet in this world's vast Underdark.