Nightmare Fuel / The Book of the Dun Cow

• Cockatrice and the Basilisks are in-universe Nightmare Fuel for the animals of Chauntecleer's Coop, especially since very little is actually seen of them on Chauntecleer's side before the battle.
• Chauntecleer's dream, which is heavily implied to be sent by Wyrm, is unsettling, especially when he believes that all of his animals died because of his selfishness. It's enough to spook him into personally checking on each one when he wakes up.
• During the first battle, the Basilisks' tactics are downright brutal - besides their deadly venom, they kill animals as large as deer by wrapping around and snapping their necks. The pain caused by their bites is enough to make animals tear their own skulls apart. Generally speaking, the violence in the book is pretty explicit.
• When the dead body of Lord Russel is reanimated as a zombie-like creature by Wyrm's evil essence in The Book of Sorrows and has to be re-killed to pacify it.
• The worms that get into Chauntecleer's body in The Book of Sorrows and corrupt him inside and out are deeply disturbing. At one point, one of them crawls out of his beak while he is talking to Pertelote.
• Chauntecleer's final battle with the wolves is brutally violent on both sides. Chauntecleer blinds Eurus with his battle spurs, slices up Fabronius's leg so badly that it becomes useless, and kills Boreas, the pack leader, in a short but bloody battle.