Donova Snowden in Winterspring wants you to defeat High Chief Winterfall.
While we still know very little about why the furbolg have been ingesting this strange substance, I do think it's best that we put a stop to it before it gets out of hand.
The Winterfall furbolg are not a normally aggressive tribe, and this change in their behavior leads me to think that they must be under orders from their chief. We most likely can't undo the damage that has already been done, but I think that if their leader is slain, they will not get any worse.
Head east to Winterfall Village.