Shades of Avalon — Carol Oates

Ben Pryor grew up as an average kid in Camden, Maine, unaware of the supernatural storm brewing in his Celtic blood. However, at nineteen, as the last born in the royal line of beings that once ruled Atlantis, Ben has eagerly embraced his newfound abilities and birthright.

When Caleb, his sister's mate, goes missing under suspicious circumstances, the prime suspect is the last remaining member of the overthrown, corrupt Guardian Council.
With the discovery that an old acquaintance has been keeping secrets, and the future Ben was so sure of shifting before his eyes, the situation becomes more complicated and the ransom for Caleb too high.

In the sequel to Shades of Atlantis, Ben will delve deeper than he ever imagined into the old, magical ways of the Guardians, the secrets of Excalibur, and the truth behind the legend of King Arthur.

What exactly did the Council hide beneath the citadel of Camelot? And can it help get Caleb back without putting the world in danger?