Warrior Cats/Nightmare Fuel

• Warrior Cats, despite generally being listed in the children's section of most bookstores, is not really meant for kids, as stated by Word of God from Erin Hunter. This really comes through in the (multiple) sections of the books featuring High-Pressure Blood, most notably Tigerstar getting ripped open and writhing in agony as he bleeds to death nine times in a row, Brightheart and Swiftpaw being brutally mauled by a pack of dogs, Sharptooth slaughtering and eating Tribe cats, Firestar's nine lives ceremony, and every battle involving Lionblaze. Nightmare Fuel just doesn't always cover it.
• The Bonehill. The. Freaking. Bonehill. Does anyone blame Firestar for freezing in terror when he sees the thing? Not to mention the scene that follows...
  • Also note that The Bonehill in Firestar's dream apparantly had some cat bones in it. Now think about that for a second, where did they get them? They couldn't have raided the graves of long-dead cats because they don't mark where they bury other cats, and if they were buried underground, they would have turned brown, and the book described them as "sun-bleached". I can only think of one answer: cannibalism.
    • Bodies left above ground will be eaten by other animals, like ants and various birds. It's not much more pleasant, of course.
• "Before there is peace, blood will spill blood and the lake will run red..." Out of all the Nightmare Dreams in the series, this one is probably the most bloody and traumatizing, and it keeps repeating itself.
• The nightmares Tigerstar sends Lionblaze of himself slaughtering Heathertail over, and over again during Power of Three. One features a river running with her blood, and in another he slices open her throat and blood comes pouring out until he is completely drenched in it. In another one, it sounds almost like he tore her open.
• Rock. Just... Rock. He's a hideous bald cat with bulging blind eyes and curled up, untrimmed claws. Not to mention the fact that he oversees a test which involves navigating through a complex underground maze in complete darkness. Oh yeah, and if it rains, the tunnels flood and you drown. Good luck.
  • Recently, he has revealled that he is doomed to live forever, aware of every bad thing that will happen, but completely powerless to change things.
• The ending of Sunrise. I'm not sure whether its Hollyleaf's insanity, including trying to murder her own mother, Leafpool being pretty much suicidal, Sol in general, or the feeling of despair that emanates from it, but he was somewhat shaken after reading it.
• In one scene, Hollyleaf imagines a mouse as Leafpool and violently tears her body to shreds, reveling in the feeling of ripping the life out of her.
• The way the Dark Forest cats train themselves in death blows: violently killing each other in front of an audience, repeatedly . In one particularly jarring scene from Omen of the Stars, the chapter suddenly cuts off with Jayfeather gagging as he hears one of these unlucky cats screaming in agony when his belly is slit open.
• Hawkfrost's death in The New Prophecy. He sets up a trap where Firestar's neck is caught in a fox trap and Brambleclaw is torn between following his beloved brother and killing Firestar and becoming leader, or betraying the brother he became so close with and nearly lost a mate over. He chooses the latter and is forced to kill Hawkfrost by impaling him with the metal spike that holds the fox trap in the ground. The next scene describes blood pouring quickly out of Hawkfrost's wound, so much so that it becomes a large pool on the ground and flows down to the lakeshore. He speaks cryptic warnings to Brambleclaw that this isn't over, which causes the blood to pour even faster, while also coughing up blood clots.
• At the end of Rise of Scourge, the picture of Tigerstar there and Scourge crawling all over his body.
• Also, whenever StarClan says they're going to give up, like in Eclipse, and the end of The Fourth Apprentice, when there's going to be a freaking war between two groups of dead cats.
• Jayfeather's vision of Dark Forest cats massacring all of ThunderClan but Ivypaw.
• The story in Code of the Clans about how Code #14 (a warrior does not need to kill to win battles) came to be. The medicine cat is visited in her dream by a very young apprentice, essentially a child, who was killed in battle the day before. It's eerie enough with him reminding her that he's dead, and speaking with a wisdom beyond his age. But then he fades away (his eyes being the last thing to disappear), giving us this line as he fades: "That WindClan warrior didn't need to kill me. I knew I was beaten. If he'd let go of me, I'd have run away. He didn't have to keep biting me, harder and harder..."