Warrior Cats/Tear Jerker

The Original Series

• Whitestorm's death at the end of The Darkest Hour turns many readers into a blubbering pile of snotty goo.
• Hell, Darkstripe's death scene. Sure, he was a traitorous piece of scum that poisoned a kit, but him dying on the ground, saying that "It's all black... There's nothing left..." was just.... Sad. Then, of course, Graystripe ruins it by saying, "Well, that's one less traitor in the forest."
• Yellowfang's death. Especially when she tells Fireheart that she wishes he was her son, instead of Brokentail.
• Forest of Secrets has Silverstream's death, which is made even worse by the ending where Graystripe leaves ThunderClan.
• Cinderpelt's lament of, "I'm never going to be a warrior, am I?"
• Stonefur's death.
• Snowkit, a tiny deaf kitten, was carried away by a hawk, all the while crying for help, with no hope of being rescued.
• Bluestar's mental breakdown after Forest of Secrets.
• Runningwind's death. Just when he was getting more screen time and looking important, he's suddenly dead.
• The death of the great warrior Lionheart, one of the noblest and pure members of ThunderClan.
• Brightheart's mauling and name-change can hit pretty hard, especially because she was such an innocent and sweet character.
• Silverstream and Graystripe's subplot is incredibly sad, mostly because it ends with the eternally cheerful Graystripe completely broken.
• Bluestar's death in A Dangerous Path.

The New Prophecy

• Feathertail's death in Moonrise.
• At the end of Twilight, Cinderpelt dies. Yes, their whole character arc of being hit by a car and never being able to be a warrior, yet managing to be a good medicine cat despite this, all while watching the cat she loves having kits with another she-cat, and remaining supportive of him despite this ends with them lying dead in ThunderClan territory.
• The very idea of the Clans leaving the forest which they had lived in for generations.

Power Of Three

• Honeyfern's death, just after a tender moment with Berrynose.
• When Ashfur died, despite his unfortunate total psychotic breakdown.

Omen Of The Stars

• Longtail's death in Fading Echoes. Briarlight being crippled, too.
• That last line in Sign of the Moon, "I will wait for you forever, Jay's Wing!"
• Firestar's death at the end of The Last Hope.
• Hollyleaf's death, which is stretched out long enough for one goodbye with Leafpool.

Expanded Universe

• In Code of the Clans, the death of Badgerpaw and his warrior name request to his mentor is quite sad.
• Mosskit's death in Secrets of the Clans.
• Bluestar's Prophecy: It Got Worse
  • Bluestar's Prophecy gives us the deaths of Mosskit, Moonflower, and Snowfur. The first's death is especially heartwrenching.
  • Bluestar's Prophecy as a whole is pretty much just one giant Tear Jerker.
    • "I'll sleep out here now, with you." ARGH. Why, Vicky, why!?
• Scourge (then called Tiny) leaving home because he thinks that he is unwanted. "Goodbye Mama"... in The Rise of Scourge.
• The Star-Crossed Lovers Ryewhisker and Cloudberry in Code of the Clans. One moment, you're pregnant and all's well, but the next your Love Interest dies at the paws of his own clanmate, all just to protect you, driving an even bigger wedge between both of your clans.
• Crookedstar's speech to Mapleshade at the end of Crookedstar's Promise. Also counts as a Crowning Moment Of Awesome and a Crowning Moment Of Heartwarming.

Mapleshade: "Your punishment is complete now, Crookedstar. You have lost everything." Crookedstar: "No, Mapleshade. You're wrong. I still have a clan that I love and am proud to lead. And now... ...now everything precious to me is here, in StarClan. My family is waiting here for me, when my ninth life has passed. It's you who have lost. You have no power over me anymore." Mapleshade: "I have destroyed you!" Crookedstar: "No, Mapleshade. I still have the cats that I loved. You have nothing and no one."

• Crookedstar's life as Crookedkit was heartwrenching. The whole scene where he is conscious of his twisted jaw and is trying (painfully) to eat outside of the medicine den for the first time but is wracked with shame and embarassment is almost too much to take. Then toss in the total agonizing descriptions of his mother's rejection of him due to his accident and it's a sobfest.
• And another one from Crookedstar's Promise after Crookedstar and Oakheart fight over how Crookedstar should treat Silverkit.

Crookedstar: How can I love her, when everyone I love dies?

• Crookedstar staying with Minnowkit and Willowkit after Willowbreeze dies and the kits are pretty much left to die from their greencough. It only gets sadder when he wakes up and Mudfur tells him that they've died in the night.