The Empty Ocean

Context

The world of Ananke is a patchwork-quilt type collage of different biomes and areas, due to its creation - simply put, it was pulled together by the monster at its centre (tentatively named "The Source" as a working title), chunk by chunk from other planets and asteroids as it floated through space, until the planet was formed as a cocoon of sorts around this monster. The planet eventually fell into orbit in an existing system. Due to the haphazard nature of this construction, expanses are plentiful in various forms as incohesive environments were forcibly mashed together.

The Empty Ocean

Covering roughly a third of Ananke's southern hemisphere, and looking like a missing puzzle piece from space, is the most unique and significant expanse on the planet - the Empty Ocean. Distant from most civilised life (possibly a generous term on Ananke), the Empty Ocean is a baffling anomaly that most folk don't bother to concern themselves with. The place is devoid of anything useful to their common lives, inhabited mainly by hostile, wild creatures and some of the insane members of a crazed cult, The Saints of the Source. As such, towns and the like tend to be further up-river or inland.
The borders between biomes on Ananke often take the form of huge rivers, and each of these eventually makes its way into the Empty Ocean. The civilised folk, who steer clear of it, therefore don't understand why it stays empty. However, the floor of the area has a number of huge whirlpools, which seem to drain the water away somewhere. While the cult revere the rivers, the empty ocean and these whirlpools (there will be more about the cult in other prompts), even they don't know the purpose of it all.
In actual fact, this entire water system is a mechanism utilised by The Source for temperature regulation and sustenance. The rivers gather nutrients from the land and its inhabitants, the whirlpools send it down through the earth to The Source, and the water then returns to the surface elsewhere on the planet at the start of the rivers, now full of a mineral by-product from The Source (More on this in future prompts).
The flow of this water, and the mineral it carries, are hugely influential on the ecosystem of Ananke, making the Empty Ocean a crucial expanse within the world and also the story of the campaign. While each expanse on the planet will have it's own impacts on to ecosystem, and many of them will have significantly more life within it, the Empty Ocean is pivotal to the survival of it all.