The theme is from this site, it is titled "The Heretic". Was this supposed to be a MIDI from The Exorcist II: The Heretic? Because I couldnt find this song in the film's soundtrack on YouTube.

http://www.cavernsofblood.com/scarystuff.html#music

Here is the direct MIDI link

http://h03.net/DontLink/Heretic.mid