The quest begins right after Jaethal arrives at the throne room to give you bad news. You are going to need to meet the elves in their camp.

Bring her into the squad and then head for the camp. You’ll find it on the left region of the area, next to the river which flows between you and Pitax.

Go there, speak with Jaethal’s daughter and determine on their destiny. Jaethal may even evaporate from the group.

This will happen only if you convince her that she has turned into a monster. This way you’ll meet Jaethal in Nyrissa’s house from the seventh chapter, and Jaethal won’t kill Tristian. More to the point, she will return to your squad.

https://www.youtube.com/watch?v=jT6uz9aSQio