Equinox (1970)

Four teenagers go on a woods hike and encounter a creepy forest ranger and a crazy old man. The old man is a scientist who had found a mysterious book bound in human skin, the Necronomicon, and when he had read its cryptic symbols it conjured monsters into existence. The teenagers keep the book, and are then pursued by monsters, a demonic cult, and the ranger. The ranger turns out to be a red flying devil in human disguise.