There are two kinds of rain that fall upon the endless fields of the Windswept World.
The first is the kind you might understand. Lost clouds wander, trying to avoid feral packs of winds and searching desperately for each other within the lonely sky. They embrace when they find each other, and over time they come together to form crowds so thick that the whole of the sky becomes a null white abstraction. When this happens, the clouds shatter under their own weight and fall to the earth in sheets, in an ecstatic warm rain that always seems to last the exact same amount of time.
The other rain is alive.
From time to time a dim, gauzy shape will make its way across the endless horizon, borne upon sleepy air currents that have yet to become wind. Up close, the shape reveals itself to be a massive, ethereal silken glider. These semitransparent shrouds bring with them a rain of tiny, floating specks that move with alien, unsettling speed and precision once they alight upon a surface.
A rain of spiders.
Left to their own devices, the tiny beings will simply vanish into the deep grasses and pass to the end of their life cycle unheralded.
But if they should happen upon a creature that moves under its own power, then that creature becomes a chariot for these tiny hunters. They will move into and infest anything they catch. And after a time, the unfortunate host will struggle, fall, and cease moving for a time - and then it will make its own way down into the deep grass.
For although the surface of the grass is level in every direction, beneath the blades lie deep chasms and trenches, some of which descend for miles. At the bottoms of those trenches are rivers of blood, blood which flows from unspeakable lakes tucked away in subterranean caverns.
In nearly every cavern, there is a Sunless Village. It is here that the spider-infested creatures have gone to hold their unspeakable congress, in the dark caverns below the sunlit grasslands of the surface, feeding upon the blood of the world.