You need to talk to Ivar to get this quest. You will find him on Snidhall Isle. To talk to him you have to kill the warrior who wants to kill him. You must act quickly so that Ivar does not die.
On one of Skellige's beaches, Geralt ran into a man desperately fending off an enraged assailant. The witcher could hardly look upon this spectacle impassively and decided to help. After the fight, Ivar, for this was the man's name, admitted his attacker had been the brother of the woman Ivar loved. He had sought to kill Ivar for seducing his sister, who was a priestess of Freya and thus bound by custom to a live a life of chastity. Ivar, however, refused to let religion stand in the way of his love and aimed to scour learned books for a precedent that would help him negotiate his beloved's release from her vows. Geralt agreed to bring him any relevant books he found.
