This is one of my favorite horror films and one of the most imaginative.
The 1987 film Prince of Darkness, directed by the master himself John Carpenter and starring Donald Pleasence and Jameson Parker. In the second film in Carpenter's "Apocalypse Trilogy," the first being the classic The Thing and the last being the mindbending In the Mouth of Madness.
The vicar of a church (played by Donald Pleasence) invites several academics to his church, including a Professor and a metaphysician named Marsh (Jameson Parker). The reason he has invited them? Because in the basement of his church, he found a hidden room where they was an ancient cylinder...filled with a mysterious green liquid. The academics try to figure out what the mysterious green liquid is, while they all seem to suffer from bad dreams.
So why is this movie so good? Well, I'm going to spoil it a bit for you: that green liquid? That's Satan. That's right: Liquid Satan. How awesome is that? The Liquid Satan starts influencing rats and bugs and homeless people (one played awesomely by Alice Cooper) and soon the vicar and the academics are fighting for the lives and holing up inside the church, hoping to survive the night. But the Liquid Satan can possess people and it wants to bring its father into the word...a being known as the Anti-God.
This movie is, very simply, a very complex and well thought out horror film, which invents new ways of scaring people. The first scene I saw of this movie was of a spurt of Liquid Satan flying into someone's mouth and possessing them...and it scared the living hell out of me. Just one scene and I was scared. And the end, my god, the end is just wonderful.