Equinox
(1970)

A cheap horror film with original visual effects from Dennis Muren who later won six Oscars for visual effects.

David, Susan, Jim and Vickie go out into the woods to look for a Dr. Waterman when they stumble upon an ancient book of supernatural power (shades of Evil Dead!).  Asmodeus (King of Demons disguised as a park ranger) sends monsters to kill our heroes leaving only one survivor, David.  A monster informs David that he is dead in one year and a day which sends David to the nut house!  David takes solace in his cross but......a year and a day later, the doctors decide that David must realize that it is just a cross and take it away from him.  David freaks out and we then see what looks like Susan approaching the asylum.

Thanks Don P!