Prince of Darkness

Prince of Darkness is a 1987 American supernatural horror film directed, written and scored by John Carpenter. Starring Donald Pleasence, Victor Wong, Jameson Parker and Lisa Blount, the film is the second installment in what Carpenter calls his "Apocalypse Trilogy", which began with The Thing (1982) and concludes with In the Mouth of Madness (1994).

Plot
A priest invites quantum physicist Professor Howard Birack and his students to join him in the basement of a monastery belonging to "The Brotherhood of Sleep", an old order who communicate through dreams.

The priest requires their assistance in investigating a mysterious cylinder containing a swirling green liquid. Among the thirteen academics present are wise-cracking Walter, demure Kelly, and lovers Brian Marsh and Catherine Danforth.

They decipher text found next to the cylinder which describes the liquid as the corporeal embodiment of Satan. The liquid appears sentient, and broadcasts increasingly 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 and attack the others. Anyone attempting to leave is killed by the growing mass of enthralled schizophrenic homeless people who surround the building.

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.

Walter, trapped in a closet, witnesses the possessed bring the cylinder to a sleeping Kelly. It opens itself and the remaining liquid absorbs into Kelly, transforming her into the physical vessel of Satan: a gruesomely disfigured being, with powers of telekinesis and regeneration. Kelly 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. Danforth, the only one free to act, tackles Kelly, causing both of them to fall through the portal. The priest then shatters the mirror with an axe, trapping Kelly, the Anti-God, and Danforth in the other realm. Danforth 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, Walter, Birack, and the priest) are rescued.

Marsh has the recurring dream again, except now Danforth (apparently possessed) is the figure emerging from the church. Marsh awakens and finds Kelly, gruesomely disfigured, lying in bed with him. This is shown to be another dream, and he awakens screaming. Rising, he approaches his bedroom mirror, hand outstretched.