In Season 2 of the Sky One series, Diana traveled back in time with Matthew to find a witch to teach her more about her magic and find out details about the Book of Life, also known as Ashmole 782. With enemies surrounding the fated couple in the past and present, their future remains at risk. 

After reading the book series, fans questioned if Diana is immortal after absorbing the Book of Life. Though it is never clearly addressed in Harkness' novels, the author answered fan questions during a Q&A about the conclusion of Diana and Matthew's stories. 

 "She’s mortal and a warm-blood, and not going to have an extended life," Harkness said. Adding, "Matthew has an extended life, and I think, again, the sad truth is none of us really live forever, and we all have to make the best of what we get handed. So I think that’s a really normal part of life. One of the things we need to do better maybe in our time is learning to say goodbye, and learning how to live without the kind of regrets."

Though fans have theorized Diana may have created her own spell, since she is a weaver, it seems the author confirmed that the titular character will live out her life as a mortal.