The Trollenberg Terror (1958)

Starring Forrest Tucker, Laurence Payne, Jennifer Jayne, Janet Munro, Warren Mitchell
Directed by Quentin Lawrence
(actor & director credits courtesy IMDB.com)

The presence of a mysterious cloud near a mountain where mountain climbers have perished persuades a United Nations agent that a terrible experience from his past is happening again. 

I love this picture, a terrific science fiction thriller adapted from Peter Key's television serial, and very well-scripted by Jimmy Sangster, better known for his Hammer horror film screenplays.  The mountain scenes, augmented by matte paintings and climbing shots that look authentic, give credibility to the alpine setting, and the cast gives fine performances, with Tucker a soft-spoken but natural hero, Munro a fascinating psychic with hypnotic eyes, and Mitchell a convivial but concerned scientist who's simply wonderful in his role.  Suspense is built steadily throughout the picture, heightened by Henry Richardson's sharp editing and Stanley Black's otherworldly music score, and when the film's creatures are finally revealed, they look wonderfully creepy and unsettling, rendered by effects that hold up surprisingly well today.