After the death of Jacaerys' mother, Queen Rhaenyra, in hands of Aegon the usurper Jace, Cregan Stark, his aunt Princess Visenya and the rest of the blacks managed to take king's landing, forcing the usurper to scape but his wife Helaena and his mother the late Queen Alicent didn't managed to do it at time and stay there as prisioners. Jacaerys was now the rightful King and he decided to take his aunt Visenya as his wife. They were on the throne room with their supporters, Alicent and Helaena