Elven Hierarch Asterios is concerned that the Mother Tree of the village is being contaminated. He asks you to visit its water source, the lake north of the village. Destroy the cause of the contamination, the plague zombies.

You've obtained plague dust from a suspicious looking zombie. Could this be what Hierarch Asterios was talking about? Take it to him.

It was a plague zombie, the spirit of a person who died from the plague. The dust from the zombie's flesh has been the cause of the contamination. A large amount of the dust is needed to perform a purification ritual. Hunt the plague zombies.

You've obtained enough plague dust for the purification ritual. Deliver it to Hierarch Asterios.