The Devil's Rain (1975), directed by Robert Fuest.
The desert satanists want their magic book back; it's been missing since colonial times when witches were burned. I fear the devout Christian family who have it will not be able to withstand them.
According to the wikipedia: "The Devil's Rain received a uniformly negative critical response, with the chief complaint being the incoherent storyline... Roger Ebert eventually added it to his Most Hated movies list".
Ok, sure, but horror fans are not much concerned with mainstream critical response. They will watch and even treasure films they admit are not objectively "good" but might still be worthwhile for other considerations.
We might shelve this one with Brotherhood of Satan (1971), another borderline-incomprehensible film that impresses with its off-kilter vision. This is more of a drive-in special effects horror feature, although it would be interesting to match it with the Esperanto demonology thriller Incubus (1966) which, as here, has William Shatner fighting for the souls of his loved ones.
As for "incoherent storyline": well, it does hit the ground running in the middle of the story, but books and films are allowed to do that.
An intriguing cast list:
Ernest Borgnine, proving he really can do anything.
Eddie Albert
William Shatner
Ida Lupino
Tom Skerritt
Keenan Wynn
John Travolta in his first feature film. I don't think he has any lines. Another cast member first introduced him to Scientology here.
Professional satanist Anton LaVey is credited as a consultant and has a small part.
The big effects draw here is a flesh-melting rain that goes on and on in the last act. The filmmakers were obviously proud of the technique and wanted to give viewers their money's worth. I'm not sure how it was done. It look like melting candle wax but is something that dissolves in water at room temperature. It also bubbles and smokes, which is good.
The "damnation" mask Shatner wears was from a casting of his face later used to make a mask that somehow found its way to Halloween (1978) where Michael Myers adopted it:
Filmed in Durango, Mexico.
Available on Blu-ray from Severin.
The commentary track is an interview with director Robert Fuest -- And Soon the Darkness (1970), The Abominable Dr. Phibes (1971), Dr. Phibes Rises Again (1972).
It is mostly an amused account of his career. For this film he says that the known stars might have considered this material to be beneath them, but no one behaved that way. All the actors were pleasant and gracious throughout.
He thinks the running time could have been trimmed, particularly the extended melting sequence in the last act.