Saw it on TV once upon a time, was probably made sometime in the 1960s. Some college kids go to visit one of their professors and find a portal to another dimension where his house used to be. The professor was an expert on medieval sorcery and witchcraft, and discovered too late that the incantation he found in an old tome really worked. The gang goes looking for hiim and find themselves in a hell-realm; I’m not sure if it was supposed to be capital-H Hell or a Doom-style “so inhospitable to human life it might as well be Hell” dimension. One by one they meet horrible fates until just one is left, and even though he gets out again he’s still doomed, as his soul is irrevocably marked for collection by the forces of Darkness. The movie ends with him in either a hospital or psych ward babbling out his unbelievable story while the possessed body of one of his former classmates is coming for him.
Ringing a faint bell - B&W or color?
That may be it, but I’d have to view it to see.