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Walkthrough
With Avallac'h turning the tables on you, you'll need to follow Yennefer to the tower at Tor Gvalch'ca (#1) so you can confront him. Along the way you'll have to dodge fireballs and ice elementals, but you'll also pass by a pair of horses, which will speed up the trip. If you have trouble following Yennefer while on horseback, then just ride for your destination and let her catch up to you.
Eventually, Yennefer will dismount, and you'll have to stay close to her while she protects you from the icy conditions. Then when you get close to the tower, Yennefer will inform you that there is a magical barrier protecting the site, and that she'll only be able to create a tear large enough for you to go through. So you'll be on your own.
When you reach the tower, you'll find Avallac'h there alone. He'll sound apologetic, but you won't much care what he has to say. Instead, you'll challenge him to a fight, but he'll throw down his sword. You'll then learn that he's been working with Ciri -- and that Ciri wants to go through the portal between worlds so she can stop the White Frost and put an end to Ithlinne's Prophecy.
No matter what you say, Ciri will enter the portal and disappear. She'll then remember five moments during the campaign: how you settled her nerves during the quest
Blood on the Battlefield, what happened with the emperor during the quest
Blood on the Battlefield, whether you let her face the Lodge alone during the quest
Final Preparations, whether you encouraged her to trash Avallac'h's lab during the quest
Child of the Elder Blood, and whether you accompanied her to Skjall's grave during the quest
Skjall's Grave. These five decisions will play a large role in determining Ciri's ending for the game.
1 - Tor Gvalch'ca