Back story
The sheer power unleashed threw Naruto for a loop. Thrown from his feet, he went spiraling through the air, landing yards away and leaving a path of destruction in his wake. He groaned and stumbled to his feet, wavering slightly and catching himself.
"Orochimaru, you bastard," He snarled, voice hinting the appearance of the kyuubi.
"Oh foolish child," Hissed the pale-faced man. "Did you really believe you would, let alone could, kill me without being half dead also?"
"Naruto!" A strained voice came behind them. He turned to see Sakura attempting to rise also and fight. Naruto moved to be at her side, but Orochimaru blocked his path, long tongue slithering past his lips, a completely vile grin on his face.
"Oh no, no, Naruto," He chuckled, brandishing a kunai knife. "I'm your opponent. You challenged me and you won't stray from this fight while I'm still breathing." The ninja licked the blade, golden eyes flicking to the snarling boy. "Unless you want her to die, of course."
Sakura gasped and attempted to run, but Orochimaru was too quick and hurled the knife at her, the blade burying deep into her back. The pink haired ninja fell, the look of shock plastered on her face.
"Sakura!" His voice was just below hysterical as he watched his best friend fall. "No!" Naruto rushed to his friend and his knees crumpled beneath him. He reached out and pulled Sakura into his arms, burying his face against her.
"Naruto..." Sakura's voice breathed slowly and the 16 year old lifted his face to look at her. A trickle of blood trailed down her chin and her usually spirited green eyes were fogged and losing light. "Since it's obvious I'm not going to do it... You have to bring back Sasuke, for everyone back home."
"Don't say that, Sakura!" Naruto said desperately, tears threatening to spill over. "We made a promise to get him back together, right?"
"Naruto," Sakura attempted to laugh, a gentle smile on her lips, but it just came out as a rattling breath. "Please don't get your hopes up. And don't lie to yourself; we both know I'm not going home." She blinked slowly, her breathing became more spaced out and her eyes became distant.
"Sakura..." Naruto blinked and the tears finally fell, spattering on her face.
"That tickles," She giggled, finally her eyes closed and her face took a dreamlike quality. "Naruto, Sasuke, we should get out of the rain!" Then, she fell limp in his arms. Naruto let out a desperate cry and pulled her to his chest, tears trickling down his cheeks as he cried for his fallen companion. Tenderly, he laid her down and stood, hands limp at his sides.
"Oh dear, what will happen now?" Orochimaru said in mock wonder, a smirk pulling at his mouth.
"You're going to die!" A jolt of red chakra pulsed across the ground, and then slowly grew. Naruto whirled, eyes flashing blood red past the tears and fangs bared. He crouched, and then leapt forward, claws reaching for him, when suddenly, something gripped him by the shoulders, restraining his arms. He howled in fury and struggled madly, not taking his gaze from the ninja that murdered his teammate.
"Naruto, stop it, you know this is just what he wants you to do." An indifferent, familiar voice said and automatically, the kyuubi vanished, leaving Naruto in a half-daze.
"Sasuke?" He turned his head as far as he could to look at his old friend.
"Who else, dobe?" He flashed a half-smile, and then let Naruto go.
"Sasuke, Sakura! She..."
"I know, but at least we can go home, for her last wish." He turned slowly and began walking forward. As usual, Naruto fell in behind, slightly in a daze, his mind not comprehending what exactly was happening. But suddenly he paused, a feeling in his stomach that something wasn't right.
Suddenly, Naruto couldn't move, it was as if all function in his limbs had stopped. He glared at Sasuke, clenching his jaw.
"I knew it was too good..." He grunted, trying to fight the jutsu claiming his mobility.
"You will always be the biggest idiot I've ever known, Naruto." Sasuke flashed a cunning smirk and Orochimaru appeared at his side, also smiling.
"I knew the only way to capture you like this was to distract you, and what better way than to kill a comrade, a person you loved?"
"Bastards..." Naruto hissed between his teeth, and then flashed a look at his once-friend. "I'll never forgive you for this Sasuke! I hope your soul rots in Hell next to his!" He spat at the Uchiha, then cast a longing look at Sakura's body, and looked scornfully away from both of them, eyes resting on the walls of the cave.
"So no regrets?" Orochimaru cooed. "That would have made this so much more fun, but oh well, what can you do?" In a flash, he made the appropriate hand signs, calling the name of the jutsu. "Living Burial: Thousand Years of Captivity!"
Crystal slowly began to encase Naruto's body, but he was done struggling. Completely betrayed and the one he had vowed to protect dead. What more was there? As the crystal engulfed his head and he lost consciousness, he thought back to his life when he was 12. All of his fellow ninja, getting stronger for different reasons, but doing it together, he thought of each of them. Kiba, Shino, Hinata, Ino, Chouji, Shikamaru, TenTen, Neji, Lee, Gaara, Kankouro, Temari. And Kakashi-sensei, Iruka-sensei Gai-sensei, Jiraiya-sensei, Old Man Hokage, Tsunade. In his last few moments, he smiled. The fox-boy had come so far from his lonely days as a child, he had gained the best friends anyone could have, a kindred spirit, and companions that knew him as more than a demon. Of course he had no regrets.
And he slipped into darkness.