It's Abby, the blaxploitation answer to The Exorcist. In fact, it was such an answer to it that Warner Brothers sued the film for being too derivative and got it yanked from cinemas. And of course, once you've lost that case you can go ahead and release a "Black Exorcist" edition of the DVD, I guess.
And yes, that is William "Blacula" Marshall as the Exorcist. Also of note is that William Girdler (co-scribe, director and producer of this film) was the man who brought you The Manitou, that Tony Curtis-starring resurrected-fetus thing classic.
If you'd like to sample the radio spot for the film, you can check it out here. It's not embeddable.
That's right, Day 20 is Blaxploitation Day. We were watching the trailer for Dr. Black and Mr. Hyde last year and before that, the aforementioned Blacula fun.