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Nightmare Fuel: Watership Down
The book version
In the book are many scenes they cut from the film, such as the "Tale of El-ahrairah and the black rabbit of Inlé", which Hazel tries to avoid having told just before their Impossible Mission because it'll demoralize, ie. scare the crap out of, everyone concerned. Bigwig insists on hearing it, though, and so we have to listen to how El-ahrairah descends into the rabbit equivalent of Hades to confront Death as a last resort to save his besieged people. El-ahrairah gradually loses his whiskers, his tail and his ears to the Black Rabbit in futile bets, then in total desperation jumps into a well of deadly plague germs in order to carry them back to the enemy - but it it won't work, as the plague is carried by fleas nesting in rabbits' ears. Finally, the Black Rabbit agrees to El-ahrairah's request simply because his persistence in remaining alive is disturbing the place of the dead. So he sends mysterious demons to terrify the enemy into submission...
"...And that is why no rabbit who tells the tales of El-ahrairah can say what kind of creatures they were or what they looked like. Not one of them has ever been seen, from that day to this."
Just about every one of Fiver's visions, Bigwig's encounter with the Shining Wire, and the poisoning of the Sandleford Warren, to name a few. The film tends toward either short and surreal (the visions and flashbacks) or realistic and gritty (all the violence portrayed in the present tense), while the book has longer descriptions. Pick your poison, and know this stuff can be disturbing to more than just kids.
When Bigwig calls Hyzenthlay up to his burrow, her initial response is hauntingly similar to a sex slave. "If you want a doe you can have her, and no-one is allowed to stop you" takes on a horrible new tone.
To the Efrafa rabbits the fact that Bigwig isn't the chief is one, due to the assumption that somehow there is a rabbit bigger and more badass than Bigwig out there.
The sequel, Tales from Watership Down, feature several new El-ahrairah tales that feature out-and-out Eldritch Abominations, from a predatory Elder Thing in "The Story of the Comical Field" to a monster beyond time and space in "The Three Cows" to something only seen in gibbering insanity in "The Hole in the Sky." Even a couple of the non-Eldritch Abomination tales are nightmare-inducing: "The Terrible Hay-Making" features a massacre even worse then what took place in Sandleford.
The animated film version
Yes it's a U-rated film (or G-rated, if you're American)...but Bigwig almost choking to death extremely graphically and realistically is horrifying.
Any of the numerous scenes that involve rabbits with blood coming out of their mouths are pure Nightmare Fuel. That and the hallucinations with the red eyes. "The field...it's covered with blood!" Another animated film from the same studio, based on a book by the same author as Watership Down, The Plague Dogs doesn't even pretend to be a family flick.
The flashback to the gassing of the warren, with the glowing-eyed rabbits crammed into the tunnels clawing each other, trying to get out. Holocaust parallels, anyone?
Cowslip's warren. It's a nice little place with good food and shelter, but then the leader does some creepy rambling about the Black Rabbit. Turns out that the warren is near a farm that leaves the food in order to fatten and trap the rabbits.
Near the end when Woundwort attacks the dog, if you're watching on YouTube, pause at the right time and Woundwort will look like something out of your worst nightmares.
The disturbing parallells between Efrafa and Nazi Germany.