I Intend of keeping my promise to her (book 6)

Two years after the fight with Arias Leon and Lena try to return to a normal life after that. Until Leon one day goes on a mission leaving Lena alone with a sitter. Little did they took down arias all except for one. One who wanted revenge for her father. That when it all starts to go wrong as they try to survive in alcatraz filled with zombies and someone whose out for revenge