Prince of Darkness is a 1987 American supernatural horror film, written and directed by John Carpenter, and starring Donald Pleasence, Victor Wong, Jameson Parker, and Lisa Blount. 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)—the film follows a group of quantum physics students in Los Angeles who are asked to assist a Catholic priest in investigating an ancient cylinder of liquid discovered in a monastery, which they come to find is a sentient, liquid embodiment of Satan.
A Catholic priest invites quantum physicist Professor Howard Birack and his students to join him in the basement of a Los Angeles 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, the highly-strung Susan, and lovers Brian and Catherine.
They decipher an ancient text found next to the cylinder which describes the liquid as the corporeal embodiment of Satan. The team also learns Jesus Christ was an extra-terrestrial who was executed for heresy after trying to warn the people of Earth about the vessel in which Satan was trapped. The liquid is then discovered to be sentient. The academics use a computer to analyze the books surrounding it, and find that they included 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. The first victim is Susan, who begins killing off the others one by one, after which they too become possessed. Anyone who attempts to flee the monastery is killed by the growing mass of enthralled homeless people who have surrounded the building.
Professor Birack and the priest theorize that Satan is actually the offspring of the "Anti-God", an even more powerful force of evil bound to the realm of anti-matter.