The Irrefutable Truth About Demons (2000)

Anthropologist Harry Ballard researches obscure demon cults and sects since his brother died, victim of one of those same demon-worshiping cults. One day he receives a threat from a so-called Black Convent, whose leader seeks to prey on Ballard. With the help of a strange, sexy woman he makes his way through the night in search of his salvation and the redemption of his lost brother.