This takes place in an alternative universe after the first 3 books, where Soren married Gylfie and had a child named Sullivan. In this story the king and queen of Ga'Hoole was assassinated and the Great Ga'Hoole Tree was overrun. Soren married Gylfie before this and they were nominated king and queen and had their son and teamed up with the wolves from the beyond and must now defeat the wretched metalbeak and his masters (in collaboration with Firefoxsnow, go follow)