Identify This (1970s?) Horror Movie

To the best of my recollection, I saw this in 1981, but certainly no later than 1982. I recall very few details, but I’m thinking it was in the style of other 1970s horror movies such as The Texas Chainsaw Massacre, Last House on the Left, etc. In fact, I only remember a fragment of one scene.

Naked guy lying on the floor (maybe in a chair beforehand; certainly on the floor after), hands bound behind his back. Ankles possibly bound together as well. Another man (I vaguely, vaguely, remember him being dressed in something much like a speedo or small black panties, and a long (dark-ish?) cape). The villain’s hair was dark, possibly curly, reasonably short (may have been a wig). The villain was wielding a sword, screamed “I am Superwoman!” (maybe “I am Supergirl!”), then chopped Naked Bound Guy’s head off with a swing of the sword. NBG is now on the floor, spasming and jerking quite a bit, and we see him from the back. There may have been other bound people in the room/house. “Superwoman” may have been hopped up on goofballs, crazy, or both.

And that’s about all I remember. In my defense, I was rather in my cups at the time, and may have walked in on the screening (at the AF watering hole on a base in the UK–the movie being shown on a portable screen from some kind of projector (well, duh)–and I wonder now what kind of medium it may have been on. Laserdisc? Dunno.) halfway through or even near the end.

I have seen questions similar to this answered in a single-digit number of minutes many times before. Wonder how long it will be before this mystery is solved (if it is)? Thanks for helping to scratch a decades-old itch in my brain!

Beyond The Valley Of The Dolls, of course.

ETA: Sorry it took so long; I just woke up.

Nineteen minutes whilst rubbing sleep out of your eyes? Good enough for me! Thanks, Snowboarder Bo! (goes to check Netflix)

“Beyond the Valley…” (from a screenplay by Roger Ebert!) was not a horror movie, it was really a melodrama, but one that was so campy it plays more like a goofy slapstick comedy.

That ending murder scene was a riff on the Manson family murders (which was still very much in the news when the film was being made in late '70.) But there’s a particularly creepy coincidence in that the “superwoman” character is a hot-shot record producer “king of the L.A. music scene” who is an unsubtle parody of Phil Spector.
Through most of the film, ‘Superwoman’ is depicted as a man (slightlly effeminate in a Mick Jagger type way), but after removing “his” wig and revealing himself to be a trans-sexual woman (real progressive, Rog!), she not only beheads the guy, but also shoots another one of her guests, a woman, in the face; something that the real Phil Spector was found guilty of doing 33 years later. And to top it off, while Spector wasn’t revealed to be a ‘trans man’, there was a big deal of his having to remove the wig he’d worn for years and display his genuine unsettling visage to the world.