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Nightmare Fuel: You Only Live Twice
The opening sequence of the spacecraft literally being swallowed up by the larger spacecraft, leaving the remaining astronaut to freeze and suffocate to death, is downright terrifying.
Blofeld punishes Helga Brandt by dropping her into a pool of pirahnas. We've already seem them strip a leg of lamb to the bone in seconds; now we see Brandt burst to the surface, scream and try to get to shore before she's dragged underneath again. What makes it worse is that we never see what's down there. Traumatizing.
Aki's death. Think of how many times you've swallowed in your sleep or slept with your mouth open. Pleasant dreams.
The Garden of Death, which really lives up to its name. At one point Bond sees one of its suicidal visitors with his head grotesquely swollen from the many poisons that the place harbours running into a pond, only to be eaten by hungry piranha inhabiting it.