Hierarch Asterios:
This ...! The undead you came across at the water source are called Plague Zombies, and are those creatures who were killed by plagues and that have been resurrected with black magic. The fine powder that falls from their decomposing bodies is this Plague Dust ... This is the substance that has been polluting our water. You would have been infected by the plague if you had not taken the Hyacinth Charm ...
Hmm ... This is definitely the doing of the Dark Elves! They cannot be forgiven! But first, we must revive the dying Mother Tree. To do that, we must purify the polluted waters ...
I will need more Plague Dust to research a purification spell. Please go back to the water source and obtain this cursed powder for me. I need at least five handfuls.