The Black Water is a chaos taint that is most concentrated within and around the trench known as the Soldieve Deeps. About ten nautical miles off the Amber coast, the pollution might be caused by the foulness of The Blighted Delta, or perhaps it is the other way around. To the south of the Deeps, a swath of the giant kelp forest has been corrupted, either by proximity or the foul work of a chaos cult. Within the trench itself, the water is visibly dark, and the creatures that lived and hunted within have become corrupted, twisting into nightmare versions of what they once were.
In recent times, corruption has spread to the far reaches of the realm in the form of tainted blood from some mystical creature drawn into the subterranean currents. This has resulted in reports of hideous manifestations of predatory organisms, which must be dealt with by the Blackwater Irregulars.
All of these things are bad news for Rebma, but the worst is at the bottom of the Soldieve Deeps trench, where Leviathan is said to slumber.