The Seventh Door by Bryan Davis

Book 3 of Children of the Bard

Summary:

An apocalypse approaches, and only Matt Bannister and his mother, Bonnie, can stop it. Forced to participate in the demon Tamiel's scheme, Matt, Bonnie, and Darcy must travel to seven destinations. Behind each of the seven doors, they find a unique brand of evil that is designed to weaken Bonnie's soul-healing song and bring about God's wrath upon Earth. 

In the meantime, Matt's twin sister, Lauren, searches for their father, Billy Bannister, who has been captured by Tamiel's soldiers. Lauren is accompanied by Walter, Ashley, and the dragoness Roxil in a desperate attempt to rescue Billy before his captors can carry out their plan to kill him.

Since Sapphira Adi has also been abducted, Elam, Yereq, Makaidos, and Thigocia travel toward a stronghold designed to keep Sapphira's fiery powers under control. Along the way, a great disaster strikes that threatens their journey as well as Matt's and Lauren's hopes to complete their missions.

By the time Matt and his companions reach the final destination, ancient mysteries are solved, dark secrets come to light, and the learn that opening the door will bring doom upon the planet. Yet, a great tragedy befalls the company, and the only remedy is to seek the power that lies behind the Seventh Door. 

Opening Lines: 

Q winged shadow flitted by the motel room window, first one way, then the other, like an animated silhouette painted on the drapes. Matt lifted from his pillow and propped himself on an elbow. Was it a bird? If so, it had to be a big one, maybe an owl sweeping past. Yet, bird or not, something had ignited the danger alarm in his gut. No more sleeping, at least not until the sensation eased. 

From the partially closed bathroom, a narrow shaft of light illuminated the area, revealing a TV, desk, dresser, and two beds. Matt lay in the one closer to the window, while his mother and Darcy slept in the other. The pairing seemed odd--a winged woman of incomparable faith slumbering next to a young prostitute fresh from a street corner--strange bedfellows indeed.