Here's an idea of mine that's been stewing in my brain for a while. It's just a bare-bones plot, mind you; I haven't ironed all the details out yet.

We begin with a prologue in the mid-to-late 1800s. We're in a Catholic catherdral. Inside the cathedral enter these shady looking characters carrying this arcane looking altar. They set the altar down on the floor and begin to chant some incantations in some otherworldly language. The head priest of the cathedral then appears and confronts them, declaring that they cannot defeat him or the power of his god. The priest then begins to recite his own incantations. The power of the priest begins to drain the very life from these invaders, but they continue with their rites. In the end they die, falling apart into ashy skeletons, but not before they complete the ritual. The priest then dies, consumed by unearthly green fire.

Jump forward to the present. The cathedral and the altar within remain intact. Then one day someone accidentally jars the altar loose from its spot and damages it in the process. At that point the sinister Black Man, a shadowy apparition in black robes with glowing green eyes and saliva, awakens and begins to afflict the parishioners and the current head priest with horrible nightmares that bring their deepest, darkest fears and desires to the front of their minds. As the nightmares persist and grow more realistic the parishioners begin to grow irritable and violent, and the Black Man feeds off these negative emotions, growing stronger as he does so.

And as he feeds the Black Man begins to interact with the current head priest in his waking life, commanding him to do one thing: destroy the altar.