The Root of the Problem
This quest given to you by Helena in Forest Encampment requires you to travel to poison the tree in the River Crossings with Maligaro's Spike and The Baleful Gem.
Maligaro's Spike is dropped by The Weaver in The Weaver's Chambers.
Baleful Gem is acquired in the Chamber of Sins Level 3.
This quest is completed once you make it to the Pools and Streams.