Been reading through The Plane Above–the parts I didn’t write, mostly ;-)–and I’ve decided on a campaign I have to run at some point in the future.
Back during the Dawn War, the gods made a pact with… Something. Something from beyond the cosmos, perhaps an abnormally sane/communicative creature of the Far Realm, perhaps something else entirely. They did when the war against the Primordials looked bleak indeed, and they were desperate. The deal was for assistance against the Primordials, which they received–but the price was that they would eventually have to sacrifice several million souls (cue Dr. Evil pinky here) to this creature. The gods made this deal through Tharizdun, the only one of their number mad enough to commune with this creature.
The gods reneged. One of their reasons for imprisoning Tharizdun was to avoid payment on this horrible debt, since the deal was technically with Tharizdun alone.
But now, ages later, the maruts have somehow unearthed word of this debt. And being the enforcers of binding law that they are, they have set about to collect. They have no access to the souls that have passed beyond the Raven Queen’s gates. They cannot enter the divine dominions. They cannot just start killing mortals in the real world, because that sends most of them to one of the above fates.
But they can cull their sacrifices from the border islands surround divine dominions, and the survivors of the various shattered/abandoned dominions. It’s from the cultures of these many "islands," the only independent cultures of humanoid souls in the Astral Sea, that the PCs come.
They are part of a ragtag fleet (here we go ;-)) of spelljammers, planar drommonds, and the like, fleeing the burning wastes that were once the border islands. The gods cannot and will not interfere for directly; the fact that the maruts are actually acting in accordance with the gods’ oaths prevents them from doing so. A few do appear to the fleet now and again, but all they can offer is guidance, as these several thousand lost souls sail the unexplored far reaches of the Astral Sea, beyond the domain of gods or the telling of myths, seeking sanctuary from the constant ravages of the marut armies, as well as the many other raiders and hazards the Astral Sea has to offer.
Yes, I spend way too much time thinking about this stuff.