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Nightmare Fuel: Spider Man 3
After being forced off of Peter, the symbiote latches on to its next host with a vengeance. Seemingly in order to ensure that it won't be rejected again, it goes all the way with Eddie Brock, literally pouring itself down his throat. But by far the creepiest part of that scene was when Brock started to enjoy it.
Also when Eddie gets pulled out of the symbiote in the final battle. Not that bit, but when the symbiote starts to grow taller, towering above Peter, nearly reaching the roof... that's when you realise that THIS is the true Venom.◊ It's a good thing Carnage was never brought into the film, because that would have been absolutely terrifying.
In the third movie, the symbiote attaching itself to Peter as he lay asleep and defenseless.