Veni Vidi Vigo
Main quest
Skellige suggested level 28 
On the world map
From the journal
As an attentive reader you surely remember that the Wild Hunt could only be defeated with the help of powerful magic, and for this reason Geralt had recruited the most powerful sorceresses in the world, both those he liked and those he utterly detested, to join his cause. When a chance arose to add Fringilla Vigo to the roster, Geralt seized it without a moment's hesitation and went straightaway to the emperor's ship, where the dark-haired sorceress was being held.
Fringilla Vigo was the emperor's prisoner, and probably would have remained so till the end of her days if not for Yennefer's pact with Emhyr. He agreed to let the sorceress go - on condition she help in the fight against the Wild Hunt.