Equinox (1970)
Equinox (also known as The Equinox... A Journey into the Supernatural, and released on home video as The Beast) is a 1970 American independent horror film directed by Dennis Muren and Jack Woods, and starring Edward Connell, Barbara Hewitt, Frank Bonner, and award-winning science fiction/horror writer Fritz Leiber. The plot focuses on four teenagers having a picnic in the canyons of California who stumble upon an ancient book containing secrets of a strange world that exists alongside humans, and consequently unleash a plethora of evil creatures and monsters.
More at Wikipedia.