Mousesack became an advisor to Duke Eist Tuirseach of Skellige, and accompanied Eist and his nephew Crach an Craite to the betrothal party in Cintra for Princess Pavetta's hand in marriage. During the celebration, he was delighted to recognize his old friend Geralt of Rivia among the new arrivals and pulled him aside to share gossip on the political machinations behind the party.
As the evening progressed, a knight by the name of Lord Urcheon of Erlenwald interrupts the ceremony. Queen Calanthe ordered the "beast" killed, but Geralt came to his defense. Mousesack advised Queen Calanthe to honor destiny's wish, but she takes it upon herself to kill Urcheon even after discovering that he was in love with her daughter and had claim to her under the Law of Surprise. Calanthe pulls him close, whispers "here is your destiny," and pulls out a knife aimed at his throat. Pavetta screams out and suddenly her abilities awaken, sending everyone flying across the room, Mousesack included. Pavetta and Duny float above the room until Geralt knocks them down. Realizing the error of her ways, Calanthe decides to honor the Law of Surprise and, in doing so, Calanthe also incidentally frees Urcheon of his curse. Duny, thankful to Geralt for saving his life, asks what Geralt wishes for as payment and Geralt chooses the Law of Surprise just before Pavetta throws up, signifying that she's pregnant.
Mousesack intends to stay and help guide Pavetta. He attempts to convince Geralt to stay as well, pointing out her immense primal power. Like it or not, Gerald is bound to the child. Mousesack advises him against trying to out run destiny. Geralt, however, doesn't believe in destiny, crediting Pavetta's outburst to a girl using her magic to stop her mother from gutting her lover. Nonetheless, the bond that will come to be between Geralt and Pavetta's unborn child will be extraordinary. Mousesack insists that if he dismisses it and leaves without claiming the child of surprise, he will unleash "true calamity" upon them all. Geralt is unconvinced and decides to take his chances.
Protecting Princess Ciri
Following the knight induction ceremony, Mousesack, Queen Calanthe, and King Eist host a celebratory feast, where they discuss the encroachment of Nilfgaard. Should they fall, Princess Ciri will take the throne. Calanthe insists that they won't fall as there won't be a war, but with Ciri being so persistent to know more, Calanthe gives Ciri her first lesson; "as in life, it's impossible to be fully prepared for battle." She's to keep her sword close and moving. Queen Calanthe, King Eist, and Mousesack then get word of Nilfgaard's arrival.
Mousesack and Sir Lazlo lock Ciri in her room and watch over her while the king and queen lead an army into battle against Nilfgaardian invaders. Lazlo reveals that protecting her is his highest honor. Mousesack tells Ciri the tale of how sorcerers used to lock little girls up in towers because the girls were said to announce the end of the human race, and so they were systematically killed. She asks Mousesack if her grandmother will be alright. He replies that she, Ciri, will be alright.
After several days away at battle, Queen Calanthe returns, but without the king, who died in battle. The queen is wounded, and Cintra is under attack. Calanthe assures Ciri that one day she will rule Cintra. As the Nilfgaardians reach the gate, Mousesack creates a magical barrier around the citadel.
As night falls, the Nilfgaardians breach the castle gates. In the face of defeat, Queen Calanthe orders the entire castle to commit suicide by poison, to spare them the fate of being tortured and murdered by the invading army. Calanthe will stay behind, but she is sending Ciri off with Sir Lazlo and Mousesack, telling her to find Geralt of Rivia, as he is her destiny. On her way out, Ciri screams, causing a magical pulse. With Ciri being escorted out, Calanthe reveals to Mousesack that Ciri is the reason Nilfgaard attacked.
Mousesack, Sir Lazlo, and Ciri escape the castle through an underground tunnel. Mousesack stays behind to fend off invading Nilfgaardians.
Present day, Mousesack has been captured by the Nilfgaardian invaders. As he is being pulled by a chain, he trips over Queen Calanthe's body. He grabs the cloth off her body and hides it under his shirt.
After being captured by the Nilfgaardians, Mousesack is brought before Cahir, who has enlisted in the help of a doppler to help recover Princess Ciri from Brokilon forest. Cahir orders the doppler to take on Mousesack's appearance. Cahir releases Mousesack from his chains only for the doppler to tackle him to the ground and shape shift into him, before stabbing Mousesack with a knife. After killing him, Cahir demands that Ciri is brought to him alive.