John Carpenter has admitted to being a big fan of the work of H.P. Lovecraft, and it shows in several of his movie. In the Mouth of Madness was the one film he consciously intended to be Lovecraftian, but the influence of his writing is clear in much of his work. One of the ones I often hear cited as extremely Lovecraftian is his underrated 1987 horror film Prince of Darkness. It certainly has that sort of atmosphere. The final scene is lifted almost directly from one of Lovecraft's writings and there is even a character named shares her surname with William Dyer's unfortunate companion in At The Mountains of Madness. Bringing the whole thing full circle I think this film might in turn have even influenced my own contribution to the Cthulhu Mythos.
It all begins when an elderly priest dies suddenly of an unspecified illness. Among his belongings are a diary that reveals his connection to a mysterious sect, and a key to the basement of a seemingly abandoned church. Another priest (played by Carpenter regular Donald Pleasance) decides to investigate and finds something which motivates him to contact physics professor Howard Birak (Victor Wong, who also worked with Carpenter on Big Trouble in Little China) for assistance.
What he found turns out to be an enormous vial containing an ominous green fluid. A research team is assembled to study the peculiar properties of the thing, with the only explanation lying in an ancient book that contains complex differential equations (which shouldn't have been known when it was written). This substance is eventually revealed to be in a way the essence of Satan, or to be more precise, something far more terrifying from which the religious concept was derived. It has been kept secret for thousands of years but now it is starting to awaken.
Strange things begin to occur, the homeless in the neighbouring area begin to surround the church. People are possessed by the fluid, transformed into zombie-like beings to perform its will, and carefully it begins working to bring about something even more frightening that will destroy our world. A desperate race against time ensues as the decreasing number of people who are not possessed try to survive long enough to figure out a way to end its sinister plans.
One thing I can tell you right off the bat is that this is a disturbing movie that has that atmosphere of dread right from the get go. Any film that can make a jar of green goo seem frightening is worthy of respect, and that's just the start. Even long before we see the goo, the atmosphere has an overwhelming sense of dread and hopelessness. When it does begin infecting people, things quickly become terrifying, and between the possessed individuals outside and the zombie-like servants within, the environment seems to gradually close in on the protagonists.
What I personally find interesting about this film, however, is it's approach to the material. The idea of taking old religious ideas and putting on a science fiction/Lovecraftian twist is a fascinating one that seems believable in a way. It also allows for an interesting dynamic in Donald Pleasance's role. His character of the priest is one who is constantly forced to question and re-evaluate his faith, but can never quite seem to let it all go. This constant conflict in his emotions is conveyed effectively and becomes a major driving force in the narrative.
Prince of Darkness is definitely a horror movie worth your time. I would strongly recommend you check it out, especially if you enjoyed the other two installments of John Carpenter's "Apocalypse Trilogy" (The Thing and In the Mouth of Madness). It is a terrifying experience that will have you compelled to follow from start to finish, and guaranteed to make you feel uneasy by the time it is finally over.