Child of the Elder Blood - Skellige
This quest will automatically begin during The Sunstone Story Quest when you visit Yennefer in the Kaer Trolde Harbor. Much like The Sunstone and Veni Vidi Vigo you will need to complete this quest in order to finish the Battle Preparations Quest.
For this quest you need to travel to a small island in western Skellige. There will be a Signpost on this island called The Pali Gap Coast which is right next to the cave that we need to enter. Once inside you'll find Ciri and Yennefer chatting, speak with Yennefer for a scene and some dialogue.
This cave is pretty linear, Yennefer will call you over from time to time otherwise your exploration will trigger all of the scenes that you need. You will discover pictures of Ciri, a giant family tree dating back hundreds of years and culminating in Ciri as well as Avallac'h's lover.
Note: This is a very short quest but with extremely important dialogue choices. You'll have two dialogue choices during this quest which will count towards giving you the "good ending" or the "bad ending" depending on what you choose. The first choice will come shortly after meeting Avallac'h's lover (pictured below).
When Ciri is having an emotional moment and says she wants to destroy the laboratory tell her to "Go for it" (dialogue option #2). This response will have a positive effect on the games ending. Telling her to calm down will have a negative effect on the ending.
Shortly after this you'll have another timed response dialogue option with Ciri. This time she asks you if you'll visit Skjall's grave with her, make sure you choose option #1 "Yeah, I will go with you" as this will have a positive effect on the game's ending. Choosing option #2 "We don't have time" will have a negative effect.
You will be given the option of fast traveling to Skjall's grave, take it and go through the dialogue here (the timed decision that takes place here has no bearing on the ending). Once you're finished with the dialogue this quest will be over and you'll return to Kaer Trolde Harbor where Avallac'h is waiting for you.