Prime Evil (1989)
In 1349 a group of monks led by Brother Seaton denounce God for
bringing the black death on to them and pledge allegiance to
Satan. The pact comes with a price as the monks must sacrifice
a blood relative every 13 years in order to remain immortal.
Flash forward to modern day New England where the monks are
alive and well and will apparently accept anyone into the fold
including homicidal janitors. George Parkman's 13 year duty
has arrived and promises to sacrifice his granddaughter
Alexandra who was sold into child pornography by her father
when she was six years old. .... A side plot is thrown into
the first fifteen minutes as a nun Angela volunteers to go
undercover in order to expose the evil order. Her initiation
consists of smashing ceramics and a change of wardrobe. Voila!
Instant satanist! She doesn't factor in the movie again until
the end. Alexandra soon becomes smitten with Father Seaton much
to the consternation of her boyfriend Bill. When every person
Alexandra has ever known mysteriously die her date with the
devil arrives. (IMDB Zeegrade)