Phantom Forest after passing through the poison log you will find Arhu and Cassandra towards the North of the map. Initiate dialog in order to learn that you will need to obtain a Stasis Fern.
You can get the Stasis Fern from Shearth, who is located in the basement of a hut found in the far south east of the Phantom Forest Swamp (after passing through poison log, NOT Hunter's Edge)
There are several ways to get the Fern, I ended up purchasing it from him through dialog.
Once you have obtained the fern, go back and give the Fern to Arhu.Alternatively:
Giving the Fern to Cassandra unlocks
Giving the Fern to neither and instead killing Cassandra unlocks
Saving and reloading your game before each of these will allow you to obtain all three