Oops I forgot to explain the actual story behind it.. I know it was mainly explained, but this is just more in-depth.

THIS ALL HAPPENS IN THE FOREST TERRITORIES; IT DOES NOT FOLLOW THE BOOKS' TIMELINE

So, Thrushpelt and Acorntail were never 'friends'; instead, Acorntail hated her, and wished to destroy her. So, when he followed her into the woods and found her having given birth to kits, he took his chance and killed her, taking advantage of her tired, weak state and the lack of other cats. He carried her body and kits (after washing off the blood and picking out her fur from between his claws) back to camp, declaring to the Clan he'd found her killed at the border and the kits were nearby, though dismissed any idea they belonged to her. He and his mate, Froststep, took them in, as their kits had recently been made apprentice and Froststep didn't mind. They opened their eyes a few days after being 'found', and were raised to think Acorntail and Froststep were their birth parents. Thrushpelt's worried spirit hung around the Nursery and the growing kits every day, having never really 'accepted' joining StarClan, rather joining their ranks but deciding, rather than staying with her deceased friends and family, she'd watch her kits grow and try to lead them as best she could. The kits were allowed into camp when they became a moon and a half, Flowerkit seeming to go by her herself, while Sparrowkit and Sagekit never left one another's sides. Acorntail watched them constantly, while Froststep often left camp to hunt or sneak some warrior duties in even though she wasn't supposed to. One day, when the kits were around five and a half moons, excited to become apprentices, they decided to play with a few leaves they'd found. While playing, Sparrowkit and Sagekit spotted Thrushpelt, as she'd finally revealed herself to them and them only. Flowerkit didn't seem to notice as her siblings ran over to Thrushpelt, demanding attention from her. She only sighed and stared down at them, smiling the slightest then disappearing. Sagekit and Sparrowkit were confused, though returned to their game. The next day, they earned apprentice names and mentors; Sparrowpaw got Larchfur, Sagepaw got Hollyfoot, and Flowerpaw got Owlstripe. They all stayed up until near-moonhigh, until they heard the Dusk Patrol returning. Scared of being caught talking outside their den, they all crept inside and fell asleep beside one another. The next day, their mentors took them out to show them the territories, though Acorntail snuck behind them and watched the small group as they scouted the land. Sparrowpaw made the 'patrol' drag, by pausing at nearly every new thing he saw to observe it. Acorntail was nearly spotted by him when he stopped to examine a small ladybug on a hanging branch, though he escaped in time. The cats paused at the Sandy Hollow around sunhigh, to practice some basic hunting moves, like how to catch a mouse and tricks to being quiet in the dense undergrowth. Sagepaw managed to catch a vole while they padded back to camp, Flowerpaw seeming discouraged by her sister's speedy rate at which she was learning. Owlstripe cheers her up and they continue back, arriving at camp about an hour past sunhigh. Sagepaw runs over to the Freshkill-Pile, dropping her vole proudly. Acorntail sneaks into camp behind her, and congratulates  her along with Froststep. They ask their 'kits' how training went. The next day, the apprentice learn battle moves and more hunting, Acorntail and Thrushpelt's spirit watching them the entire time. Not much happens while their apprentices, beside the medicine cat passing and a new one being named, along with Sagepaw showing the strongest hunter between the three and Flowerpaw the strongest fighter. Sparrowpaw lacks behind them but is brave and strong-spirited. About nine moons after them being apprentices, they all earn warrior names; Sparrowheart (changed from original script) for his strong spirit, Sagepounce (was changed from original script) for her good hunting abilities, and Flowerclaw (changed from original script) for her strong fighting abilities. The next day, Sparrowheart and Sagepounce left camp to hunt, and they find Froststep's body. It has a few bloody wounds, with Deathberries beside her, and a strange white substance around her mouth and on the ground beside her head. Acorntail's fur is between her claws, though picked out mostly. They are horrified, and turn tail to run back to camp, in hopes of alerting the Clan. They beg Redstar to believe them and follow them, though he dismisses them as still have strong imaginations, as they were only young warriors and younger cats often saw things and thought they were something else. As Redstar forces them out of his den and follows them out, Acorntail pads into camp, holding Froststep's body. The Clan, including a panicked Flowerclaw, all surround her body and mourn. Acorntail hurries over towards Sparrowheart and Sagepounce. In a panicked rage, Sparrowheart lashes out and causes a large gash, from above his right eye, down between them, and ending at his left cheek. Redstar becomes enraged and tackles Sparrowheart, growling. Sagepounce watches in terror, unable to move. Sparrowheart kicks the leader off, hopping onto his paws, him and Sagepounce fleeing the camp as the mourning Clan, and Flowerclaw, watch in shock and confusion, unaware of what'd just happened. The two race through the territory, crossing the fallen log over the river, and disappear into the bushes around Fourtrees, to escape up into WindClan's hills and out into the fields of corn beyond Barley's farm. Thrushpelt sits at the edge of the river, though soon tries to follow them, another StarClan spirit (Froststep) appearing beside her with an upset expression. Thrushpelt widens her eyes, as Froststep informs her she cannot stay any longer. The two return to StarClan, Thrushpelt depressed as her kits flee, never to return to ThunderClan again. Froststep is welcomed by her friends and family again, as the upset mother pads away, disappearing into starry ferns. The story ends here.