Aquilae

From website:

It seems a typical fantasy roleplaying world. Orcs, elves, gnomes, and dwarves, with a smattering of more interesting, newer races.

But prick beneath the surface—literally—and much is revealed.

Measuring roughly the size of Eurasia in total size, Aquilae is in fact a two-sided “lozenge” floating through space. On its “light” side is a normal fantasy realm. Its “darkside” has long been suppressed, its chaotic and often evil denizens born to the dark and abhorrent of the light side.

In between lies a complex network of Fissures, an underground network of tunnels that has its own measures of civilization.

And buried in the deep, deep dark of the light-side’s waters is an entire aqueous undersea kingdom.