It's a real shame this turns into such an eye-roller in the third act cos speaking as someone who was involuntarily committed for a night due to a suicide attempt the beginning is scary as FUCK.
All-glass psychiatric institutes with sliding Death Star doors. Hypnosis mind-melds. Biblical plagues of locusts. Hidden mystical cities. Globe-trotting from South America to New York City to Africa to Washington, D.C. Joyrides with demons. John Boorman pioneering POV speeding demons before Sam Raimi perfected the form. James Earl Jones barfing up a leopard and wearing a grasshopper costume. Yes, this film is baffingly, gloriously insane. Make no mistake, Exorcist II is awful, but you won't spend a single second bored. I'm so grateful it exists.
At one point, star Richard Burton - quite possibly completely blotto - says it best: "Horrible. Utterly horrible...and fascinating."