A priest invites Professor Howard Birack and his students to join him in the basement of an abandoned Los Angeles church. He requires their assistance in investigating a mysterious cylinder containing a swirling green liquid. Among those present is Brian Marsh, a student in theoretical physics.
They decipher text found next to the cylinder which describes the liquid as the corporeal embodiment of Satan. The liquid appears sentient, and broadcasts increasing complex streams of data. The academics use a computer to analyze the data, and find that it includes differential equations. Over a period of two days, small jets of liquid escape from the cylinder. Members of the group exposed to the liquid become possessed by the entity, which uses them against the others. Attempts to escape the building are thwarted by a mass of possessed street people who surround the building and barricade the doors from the outside. Two members of the research group are killed.
Birack and the priest theorize that Satan is actually the offspring of an even more powerful force of evil, the "Anti-God", who is bound to the realm of anti-matter. The survivors find themselves sharing a recurring dream (apparently a tachyon transmission sent as a warning from the future year "one-nine-nine-nine") showing a shadowy figure emerging from the front of the church. The hazy transmission changes slightly with each occurrence of the dream, revealing progressively more detail. The narration of the transmission each time instructs the dreamer that they are witnessing an actual broadcast from the future, and they must prevent this possible outcome.
Eventually, the cylinder opens and the remaining liquid is absorbed into the body of student Kelly, who transforms into the physical vessel of Satan: a gruesomely disfigured being, with powers of telekinesis and regeneration. Satan attempts to summon the Anti-God through a dimensional portal using a mirror, but the mirror is too small and the effort fails.
While the rest of the team is occupied fighting the possessed, Kelly finds a larger wall mirror and draws the Anti-God's hand through it. Marsh's lover, Catherine Danforth, is the only one free to act: she tackles Kelly, causing both of them to fall through the portal. The priest shatters the mirror, trapping Kelly, the Anti-God, and Danforth in the other realm. Catherine is seen briefly on the other side of the mirror reaching out to the portal before it closes. Immediately, the possessed die, the street people wander away, and the survivors (Marsh, Birack, The Priest, and Walter) are rescued.
Marsh has the recurring dream again, except now Catherine, apparently possessed, is the figure emerging from the church. Marsh awakens and finds Danforth, gruesomely disfigured, lying in bed with him. This is still part of his dream, however, and he awakens screaming. Rising, he approaches his bedroom mirror, hand outstretched.
Prince of Darkness was shot in Los Angeles, California in 30 days. Carpenter became inspired while researching theoretical physics and atomic theory. He recalled, "I thought it would be interesting to create some sort of ultimate evil and combine it with the notion of matter and anti-matter." This idea, which would eventually develop into the screenplay for Prince of Darkness, was to be the first of a multi-picture deal with Alive Pictures, where Carpenter was allocated $3 million per picture and complete creative control.
Executive producer Shep Gordon was also manager to singer Alice Cooper, and suggested Cooper record a song for the film. Carpenter also cast Cooper as one of the homeless zombies. Cooper allowed the "impaling device" from his stage show to be used in the film in the scene where Cooper's character kills Etchinson. The song Cooper wrote for the film, also titled "Prince of Darkness", can be heard briefly in the same scene playing through Etchinson's headphones.
Carpenter cast people that he had worked with previously, including Victor Wong, Dennis Dun and Donald Pleasence. It was Peter Jason's first film for Carpenter, and would afterward become a Carpenter regular.
The film was shot with wide-angle lenses, which combined with anamorphic format to create a lot of distortion.
Carpenter wrote the screenplay but was credited as "Martin Quatermass", which along with the name of Professor Birack's institution ("Kneale University") was an homage to British film and television writer Nigel Kneale and his best-known character, Bernard Quatermass. The story features elements associated with Kneale, including a confrontation with ancient evil (Quatermass and the Pit and The Quatermass Conclusion), messages from the future (The Road), and the scientific investigation of the paranormal (The Stone Tape). Kneale was displeased with the homage, fearing that viewers might believe that he had something to do with the film.
The poster for Prince of Darkness was created and designed by Henry Rosenthal, who worked for print production vendor Rod Dyer.
According to Carpenter in the DVD audio commentary, the post-production was done at the Walt Disney Studios in Burbank, California.
In an interview with Michael Doyle in the November 2012 issue of Rue Morgue, John Carpenter revealed how he created the eerie dream sequences in Prince of Darkness that feature a shadowy figure emerging from a church doorway. Carpenter first shot the action of the figure (played by actor Jessie Ferguson) with a video camera and then "re-photographed it on a television set" in order to give the image a peculiar, dislocated feeling that also appeared as if it was being filmed live. Doyle also reminded Carpenter that the director himself provided the disembodied voice that narrates each dream.
Prince of Darkness was poorly received critically upon release. In his review for the Washington Post, Richard Harrington wrote, “At one point Pleasence vows that 'it's a secret that can no longer be kept.' Here's another: 'The Prince of Darkness stinks.' It too deserves to be shut up in a canister for 7 million years". Liam Lacey, in his review for the Globe and Mail, wrote, “There is no character really worth caring about, no sympathy to any of these characters. The principal romantic couple, Jameson Parker and Lisa Blount, are unpleasant enough to create an unfortunate ambivalence about their eternal destinies”. In his review for the New York Times, Vincent Canby called the film a "surprisingly cheesy horror film to come from Mr. Carpenter, a director whose work is usually far more efficient and inventive."
The film currently holds a 58% rating on Rotten Tomatoes based on 23 reviews.
In 2004, Jim Emerson wrote that Prince of Darkness was an undervalued horror film: "What makes me goose-pimply about Prince of Darkness is its goofy-but-ingenious central conceit and its truly Surrealistic imagery, some of which could have sprouted out of Buñuel and Dali's Un Chien Andalou."
Like most of Carpenter's films, Prince of Darkness went on to have a cult following.
In 1988, the film was nominated for a Saturn Award for best music, and won the Critics Award at the Avoriaz Fantastic Film Festival.
On September 24, 2013, the film was released by Shout! Factory on Blu-ray and DVD as part of the Scream Factory line up.