Pro tip: don't put King Crimson in your opening credits cos nothing else in the film is gonna match that for mournfulness and creeping dread.
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."