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!