The once and future king?

Question

Can someone tell me the significance of the the tittle of book two " The queen of air and darkness"

Answer 1

I have not read the book, but here are a couple of links that may help you.  

Answer 2

It is about Morgause!

Morgause travels to Arthur’s court, supposedly to reconcile Arthur with Lot. While at the court, she uses magic to seduce Arthur. Arthur is not aware that Morgause is his half-sister, but the incest is still a great sin, and by sleeping with her, Arthur ultimately brings about his own destruction.

Thats what I think:)