Requiem - Sabrina Strong Series, #6

Bill Gannon is back from the dead, and resumes his mission to protect Sabrina - the last Sibyl. When the vindictive demon Naamah enlists a terrible witch in her quest for retribution, Sabrina is in danger, and the secret behind a mysterious newcomer is revealed. With her protectors facing a powerful enemy, can they find a way to save Sabrina before it's too late?