"Messiah of Evil" isn't one of those movies that gets talked about a lot, even by cult movie enthusiasts. Made by Willard Huyck and Gloria Katz (who would later go on to infamy by making "Howard the Duck"), it was released in 1971 to a sort of general disinterest. Which is a shame, because it's a surprisingly creepy and chilling little movie whose atmosphere of dread lingers long after you've finished watching it.
The plot concerns a young woman named Arletty (who's narrating the entire thing from inside an insane asylum) who moves to the tiny town of Point Dune to find out what happened to her father. He was a painter who lived in Point Dune, but he's gone missing, leaving behind only a cryptic diary that hints at bizarre occult rituals and hideous physical transformations. Arletty moves into his old house, and winds up giving crash space to Thom, a folklorist who came to Point Dune to investigate tales of a "dark stranger" who visited the town a hundred years ago, and to Thom's two friends/girlfriends/hey-look-it's-the-Seventies, Toni and Joanne. Arletty finds herself attracted to Thom, but isn't sure she wants to get involved in the complex dynamics of his relationships.
As time passes and the hundredth anniversary of the arrival of the stranger approaches, the townsfolk become increasingly creepy and murderous. Joanne and Toni both die two of the best deaths in horror-movie history (Joanne finds out that supermarkets at midnight are exactly as eerie and unnerving as you'd expect, while Toni goes to see a movie in a deserted theater, and is so engrossed in the film she doesn't even notice it slowly fill up with the transformed townspeople. By the time she does...) Arletty slowly succumbs to the same process that changed her father, and although she hasn't yet become insane and murderous, she knows it's just a matter of time.
Finally, things come to a head as the townspeople attack. Arletty and Thom are surrounded, and are forced to swim for it, but then Thom is pulled under. Arletty gives up and swims back to Point Dune, just in time for the dark stranger to return from his home underneath the waves and claim her as his bride. Afterwards, she is allowed to live and to spread her tale of the coming evil, but only because the messiah knows nobody will believe her...
The thing that makes the movie special is the way it understates. Key plot points are only hinted at, giving the film a dream-like atmosphere of paranoia and mystery that stays with you long after the credits. The dark stranger's face is never glimpsed...but he looks tantalizingly similar to Thom, and only appears after Thom is pulled under the waves. The transformation of the townspeople is different enough from typical horror-movie zombies/vampires/werewolves to be haunting. And the use of familiar settings to accentuate the horror is absolutely brilliant (one character is murdered under the neon lights of a 24-hour gas station.)
This one's worth checking out, with the understanding that it's not going to give you the answers a conventional horror film might. It's Lovecraftian without actually being a Lovecraft adaptation, which alone should be enough to entice some people into watching it.