Having a Metascore of 50 is unjustifiable, because Prince of Darkness is a smart and original film and a strong example of high concept/low budget filmmaking. Carpenter mixes a touch of sci-fi with gothic and urban horror and religious lore, concocting a plot that's like something out of an Alan Moore Swamp Thing or Hellblazer.
Carpenter was inspired to write Prince of Darkness after researching theoretic physics and atomic theory. The story centers on a mysterious cylinder full of a sentient green liquid that has been protected and kept secret by the Brotherhood of Sleep, a group of monks who's actual function is even a secret from the Vatican. The liquid is actually Satan himself and he's been imprisoned there by his father, the Anti-God, a being bound to the realm of anti-matter (an idea reminiscent of DC Comics Anti-Monitor).
A priest played by Donald Pleasence asks a professor and his physics students to come to an abandoned church to investigate the cylinder and try to determine it's origins and content. The liquid starts to escape and infect people including the homeless community around the church, turning them into zombies who trap the team inside, while the liquid begins to work on the team itself. Meanwhile everyone is having Villains Wikia's entry on PoD for a more in depth explanation on the Anti-God).
Prince of Darkness is such a cool and out-of-left-field concept for a movie that on it's surface almost plays like a supernatural Assault on Precinct 13. The first time I rented it I had to rewatch it again that same afternoon, to fully understand what I just saw and I was still a little unsure of everything. My only complaint is that Carpenter didn't have double the budget to more fully realize some of those bigger ideas.
Also worth mentioning is Alice Cooper's appearance as one of the zombified homeless people. He adds to the already significant creep factor with his wordless performance.