The Exorcist Code 4: Possession

The Exorcist Code #4

by William Massa

Demonic possession is evolving.
Father Callum and android exorcist Michael face technological horrors beyond imagination.

Discover a series readers are calling a blend of THE EXORCIST and EX MACHINA.

The war between Heaven and Hell has entered a new phase. The Necrocist—demonic serial killer of exorcists—has joined forces with the ghost of a visionary madman obsessed with transcending human biology. Their the perfect vessel for a demon, a nightmarish fusion of flesh and technology.

As faith fractures and technology spreads into every corner of human life, possession is no longer an invasion. It is an upgrade.

Perfect for fans of The Pope’s Exorcist, Sinister, and The Conjuring. A must-read for readers of occult thrillers, exorcism fiction, supernatural horror suspense and science fiction horror.

Genres: Horror

192 pages, Kindle Edition

Published June 30, 2026

About the author

William Massa is a produced screenwriter and bestselling Amazon author who writes monsters, magic, and mayhem with a cinematic edge.

His screen credits include Return to House on Haunted Hill, and he has sold scripts and pitches to Warner Bros., Sony, USA Network, Silver Pictures, Dark Castle Entertainment, and Maverick Films.

Drawing on a lifelong love of horror, action movies, pulp adventure, and the supernatural, William specializes in Dark Urban Fantasy, Action-Adventure Horror, and supernatural thrillers—a blend he likes to call Heroic Horror: ordinary and extraordinary heroes facing impossible evil and fighting back.

His series include The Hexecutioner, The Paranormalist, Shadow Detective, Night Slayer, Occult Assassin, and The Exorcist Code, with new nightmares always in the works.

William has lived in New York, Florida, and Europe and now resides in Venice Beach, California, surrounded by skaters, surfers, palm trees and plenty of inspiration for things that go bump in the night.

Find William on social media.

Facebook: WilliamMassaBooks for news, updates and announcements.

Instagram: @bill.massa

Email: williammassabooks@gmail

Reviews

[Kimberly · Rating 3 out of 5]

I really enjoyed this entire series of THE EXORCIST CODE, by William Massa. That being said, this fourth, and possibly final, installment was my least favorite in terms of plot and ending. It felt more "rushed" than the previous three, and the ending itself just left me underwhelmed. It was fast Paced, but perhaps "too" fast, as any further character development or deepening of the plot were nonexistent. Having read all of the preceding books, I just expected something more in this one.

Still, I recommend this entire series if you're looking for a unique possession theme.

[Patrick Dorn · Rating 5 out of 5]

The four-part Exorcist Code series comes to its harrowing, thrilling conclusion as a battered, burned-out exorcist and his android partner take on threats from THREE cyber-supernatural threats. There's the ghost of a sadistic mad scientist whose work in cybernetics allowed him, upon death, to literally become a "ghost in the machine." There's an AI program that was used to train the android exorcist that now thinks and acts as if it's a real demon, and there's the Nexorcist, an actual demon serial killer of exorcists who has successfully hacked into the digital realm. That's a lot of sci-fi/horror adversaries, but these villains have been introduced gradually over the past three books in the series, so I recommend you read those first. From a routine exorcism that goes terribly wrong in Bavaria, to a diabolical "human enhancement" facility in the jungles of Colombia, and then finally confronting the ultimate evil in the halls of the Vatican itself, Father Callan and his synthetic partner Michael have become a team, despite previous doubts. When evil and the demonic take over technology, technology and humans can work together for good, to restore balance. But, as one of the defeated supernatural villains says, "You still think death matters." Which means there could be a return, even though all the loose ends are nicely tied up and sealed in this conclusion. Like most "exorcism horror," evil is very real and malevolent, but the power and victory of God and the name of Jesus are conveniently ignored, even by the clergy, leaving humanity outgunned and overmatched at every turn.

[Julie R · Rating 5 out of 5]

Absolutely thrilling! But I'm hoping the series would start again . There are more souls to save

The final battle. A possessed priest, locked in a room the all the Vatican exorcists. Only one problem and it's huge! Father Callum is possessed by the demon Necrosist, whose only purpose is to kill exorcists. Who will win the battle? Good or Evil ? To find out you have to read this page turning thriller!

[william m knox · Rating 5 out of 5]

Maybe its not really ever over

In a world where evil exists, there is good to offset that evil sometimes the evil comes for the inside of the very good sent to fight it The good father and Michael have been tested and the evil is gone for now

[Trevor · Rating 4 out of 5]

Father Callum and Michael again face off against the Necrocist. This time the stakes are upped and Callum will have to face his greatest fears. Good conclusion to the series.

[HB · Rating 5 out of 5]

Exorcist Series

Enjoyed the series. It was.a lot different than I thought it was going to be. Love William Massa's books! Would recommend them