Dreadmist Peak to the northwest of the Crossroads is most likely a primary candidate as to the source of the spread of the poison. Its airy peaks have pools of water that influence the weather patterns. I suspect the water is now tainted by the demon-worshipping Burning Blade.
Use this sampler to draw water from the pool at the very top of the peak. Once filled, deliver it to Tonga Runetotem at the Crossroads. He is the one that requested our aid, and you will work with him to address this malady.