A long time ago, a cataclysmic event struck Ashenfall. It split the land and poisoned the air, sending clouds of tainted dust up into the air where it was caught by Guthix's veil. Over the years this dust has seeped into the water of Ashenfall, tainting it with a dark sort of energy. Now, if someone drinks from this tainted water, and their ancestors hadn't spent their life being slowly warped by the strange anima of Ashenfall, they build up Soulscourge, as the anima contained within their soul is damaged.
A soul is a container of anima within a person, shaped and enriched by their experiences. It is not piety, nor devotion, that grants a strong and robust soul, but adventure. Heroes, wanderers and explorers have strong and durable souls, allowing them to withstand the Soulscourge buildup for longer.
Should someone gain enough Soulscourge buildup, they will be afflicted by the condition of the same name. Their soul rips, tears and starts to unravel, dealing constant damage until it can be healed. This can be fatal, as one needs an intact soul to survive.
Fortunately, souls heal surprisingly quickly. Soulscourge buildup will fade over time, as the soul repairs and enriches itself with the adventures of Ashenfall.
To avoid Soulscourge buildup from drinking water, you need only boil it at a campfire. The taint in the water can be separated from it by boiling it and letting it evaporate off, where it harmlessly disperses into the atmosphere.