A mysterious force, causing death and decay to everything it touches, killing a number of Ordo Magica mages in SY05. Tomb of Dying Dreams, Part I.
The party were attacked by it in SY06. Tomb of Dying Dreams, Part II, and ultimately lost the Vile Water to it. The Creeping Darkness was victorious, the power of the Vile Water causing it to grow ten-fold and expand to control the land around King Hurian-Lo Apak's tomb and the nearby Malgomar river.
This current state of affairs makes travelling along the river near impossible.