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Tear Jerker: Warrior Cats: Dawn of the Clans
Since this is the new Warrior Cats arc, you should keep your tissues handy as you read.
The Sun Trail
In Sign of the Moon, Jayfeather leaves the past thinking that things will be better off now. Reading the beginning of The Sun Trail and seeing just how bad things are for the Tribe is hard, to say the least.
Fluttering Bird's death. We may not have known her very well, but she was just a kit with her whole life ahead of her. It gets worse when Quiet Rain and Gray Wing start blaming themselves. Then we have to experience it all over again when he see the enthusiasm drain out of Clear Sky and Jagged Peak.
When Gray Wing sets out to find Jagged Peak. It's not enough for him to be leaving the only home he's ever known. The ice bridge he's crossing collapses under him, leaving his mother Quiet Rain unsure whether he's alive or dead. He tries to tell her that he's alive, but there's no way of knowing if she heard him.
When Bright Stream dies. Clear Sky thought that he would be with her in his new home, but he finally realizes that he shouldn't have been so obsessed with himself. Also, the way Gray Wing blames himself, and thinks that Clear Sky hates him, is depressing as well.
Shaded Moss's death and funeral. He died just a short walk away from the new territories. The cats sharing their memories of him makes it even worse, and then Clear Sky has to add in Bright Stream...
On the first time reading through the book, Gray Wing accidentally killing Fox is a Tear Jerker because of the despair Gray Wing feels over killing another cat. The second time around it becomes even worse, because now you know that Fox isn't just a random jerk. From the bonus scene, you know his hopes and dreams, and how absolutely crushing this will be for Petal. Ouch.
Turtle Tail's departure. Gray Wing doesn't even understand that she loves him, and can't say the words she needs to hear. Storm getting together with Clear Sky afterwards hammers in that Gray Wing made the wrong choice.
Gray Wing telling Clear Sky to call after Storm. Clear Sky clearly wants to, but he can't or he'll betray his principles. It just shows how broken everyone is.
Storm's death. With her last words, she asks Gray Wing to tell Clear Sky that she's sorry. It's like Bright Stream all over again, but even worse.
Rainswept Flower's nightmares after her father dies. It hurts.
Moon Shadow's death and subsequent funeral.
When Clear Sky kills Misty, only to realize that she was just protecting her kits.
Bumble's death. Holy crap, Bumble's death.
The First Battle
Thunder and Lightning Tail finding Turtle Tail dead. The way Lightning Tail pleads for her to get up is heartbreaking.
Turtle Tail's death in general, really. After finally becoming mates with Gray Wing and making him happy, being one of Thunder's few allies at first and the scene where they watch Wind Runner's kits being born (which Thunder sadly recalls upon finding Turtle Tail's body)... Turtle Tail's death is enough to rip anyone's heart out.
Emberkit's death. Wind Runner is completely consumed by grief, and when Gray Wing tries (poorly) to comfort her, she screams at him that he's as bad as Clear Sky and tells him that when he loses someone, she'll throw his words about them "being in a better place" right back in his face. It gets worse when the Moor Group finds out that Turtle Tail is dead and Wind Runner regrets what she said.
All the cats who died over the arc briefly coming back at the end of the book to deliver a message, including the those who died in the battle, Turtle Tail, and Emberkit. Even though you know it's coming, it brings up tears when all these cats reunite with their living friends and family.