Location: Phantom Forest (after poison log)
Pre-Requirements: Having Jahan in your party.
In the far south east of the swamp area, there is a hut filled with enemies, once you have cleared them out enter the basement through a hatch located in the hut.
Here you should find Shearth (whom you can obtain the Stasis Fern from for other achievements) and the demon Balberith.
Initiate dialog and conclude in any way that does not end up with you losing Jahan (either sacrificing Jahans magical powers through dialog or fighting the demon will work).
The achievement should unlock once either of the latter options is finished.