Okay, I found this in the same bargain bin as The Slender Man and it looks like it's by the same company. So let's dive right in.
This is Cruel Embrace, a 1971 film by HORMA Studios, starring "Seann Barbour" as the main character. Still no director listed - is this going to be a thing with all HORMA films? Are they directed by a pool of directors who take no credit? I recognize some of the actor names from The Slender Man, so perhaps they did have a pool of actors, but some I don't recognize and Google gives me nothing.
The plot is this: Steward Joseph (Barbour) is a transfer student at a university where there are some mysterious happenings going on. Strange rituals, weird cults that appear at night during student celebrations, blood sacrifices. Stewie acts like a good amateur detective and investigates along with his girlfriend/gal friday Crystal.
Here there be spoilers: Stewie and Crystal figure out what's going on, that there's a cult made of up professors who sacrifices students to their god "The Archangel." They try and stop them, but Crystal ends up being killed and...this is where the movie goes off the rails for me. The Archangel itself appears as Crystal - which was, I'll admit, a very neat twist - and reveals to Stewie that it is the afterlife. Like, whenever someone dies, they go to the Archangel. No matter what. It contains all the dead. "I contain multitudes," the Archangel says. "Embrace me."
Stewie doesn't, of course, since he's the hero. But then...that portal from the end of The Slender Man appears and the "slender man" (played by Dooling again, I think) appears and extends a hand towards Steward. And Steward takes it.
As a sequel to The Slender Man, it doesn't really work. It has a completely different atmosphere, much more realistic and bleaker. And, really, it holds up on its own enough. It has good characterization, good acting, and some very good plot twists. But for some reason, the director decided to directly connect it to the previous film. I don't know why.
It didn't help that my copy of the film contained a bunch of glitches. Oh well, onto the next film!