Frozen, Sakura just stood there, unable to tear her eyes away from the three Tomoe spinning hypnotically in the girl's scarlet eyes. It was as if there was some invisible force holding her eyelids open, forcing her to keep staring into those bottomless eyes…
'Enemy Genjutsu!' screamed Inner Sakura. 'Cycle your chakra, or you're dead!'
Sakura's muscles spasmed.
Slowly but surely, she raised her trembling arms to chest height, struggling with every ounce of her mental strength to reclaim control of her body. Suddenly, Sakura felt the eyes' pressure lessen, and her hands came together in a flash to form the tiger seal.
"Release!"
Clarity abruptly returning to her mind, Sakura decided to gamble on her own Genjutsu prowess in a split-second decision, launching from the Tiger Seal into a series of hand signs, before clapping her hands.
"Reverse!"
Gaze long enough into an abyss, the abyss will gaze back into you.
It is said that eyes are windows into the soul, and that the Dōjutsu known as the Sharingan is like a mirror; an outer reflection of an Uchiha's soul. To cast a Genjutsu is to leave one's mind and touch another, so Sakura gave the Sharingan chakra residing in her own brain's chakra pathways a tug— reversing the situation and pulling the mysterious Uchiha girl into a Genjutsu of her own!
"Gotcha!" crowed Sakura, as imaginary tendrils and roots burst through the floor and snaked around the girl's limbs, immobilising her. "Now, tell me who you really are!"
All in all, their back-and-forth mental battle had lasted a little under a second in the outside world.
"I have no name," said the Uchiha girl dully. "So, they call me Nanashi."
The girl tried to wriggle free, but Sakura's proficiency with Genjutsu far surpassed hers. Sakura had complete controlled over her opponent's chakra, so even if she performed a Genjutsu release, nothing would happen!
"Don't try that again," said Sakura warningly. "Your chakra won't move without my say-so, so forget about resisting!"
"Is that so?" said Nanashi coolly. "Then, what if I do this?"
To Sakura's complete and utter shock, the three Tomoe in the mysterious Uchiha girl's irises spun and coalesced into one, before separating into three distinct crescent moons, orbiting her pupils… Sakura unconsciously reeled away from her opponent, stumbling over her own legs as the Uchiha cast her out of her mind with a mere thought.
Mangekyō Sharingan!
"Judging from your reaction," said the Uchiha girl, "it seems you already know what these eyes mean for you."
The last time she had gone up against a pair of Mangekyō Sharingan, Sakura had only won because Itachi's attention had been solely focused on trapping his little brother into his Tsukuyomi. Had they fought seriously, she would have been cut down in an instant!
She didn't have time to construct a complex inner world in the fashion of the Kurama clan, and neither did she have anyone covering her while she prepared her Genjutsu. Facing the Mangekyō Sharingan, Sakura's only hope was to resort to Genjutsu: Breast Illusion, but such a technique wouldn't entrance a woman, only obstruct her vision!
And so, Sakura turned tail and bolted.
It was the only logical choice she could have made— to face an Uchiha clan member in single combat was a death sentence, and that went doubly so for those few who had managed to evolve their eyes! She had no clue which pair of special abilities were sealed inside the Uchiha girl's eyes, so until she managed to figure out what they were, there was no chance in hell she was facing her head on!
'Who the hell is she!? And why is she picking on me!?' shouted Inner Sakura. 'How can there be a survivor!? Itachi, Obito, you missed one! I don't understand, I don't understand, I don't understand!!'
Sakura burst out of the dilapidated hut and sprinted out into the open, her enemy hot on her heels. The sharp whistle of shuriken resounded behind her, so Sakura dropped low just in time, the more-or-less deadly throwing stars soaring past her head.
"Hup!"
It was with a heavy heart that Sakura elected to make use of one of the very few ninja tools she still brought on missions, a smoke bomb. While she usually resorted to Ash Pile Burning for smokescreens, it would have been monumentally stupid of her to hide in a cloud of (extremely volatile) gunpowder, since her opponent was from the Uchiha clan and doubtlessly possessed an arsenal of Fire Style Ninjutsu that could ignite it…
Poof!
Upon making contact with the gravel of the beach, the smoke bomb exploded, releasing a large cloud of pure white smoke. Now, as long as Sakura refrained from manifesting chakra outside of her body, she would be momentarily hidden from her enemy's sight, as even the Mangekyō couldn't come close to the Byakugan when it came to seeing through things!
"Sakura, where are yoooou?"
"Sakura-chaaaaaan!"
Sakura's heart sank— those were Ino's and Naruto's voices. They had come looking for her at the worst possible time!
"Stay away!" Sakura roared. "Go back to the village, find Anko-sensei, and then leave without looking back! Whatever you do, don't come over here, or you'll die! I'm serious, this time!"
Out of the corner of her eye, Sakura caught a shadow flickering towards her through the smoke, swinging a kunai at her throat— by raising her voice, she had revealed her location in the smokescreen to her foe!
Sakura pre-emptively summoned her Chakra Enhanced Strength and stamped on the ground, shattering the gravel under her foot and creating a large depression in the beach. Using the recoil from her stomp, she backflipped out of the smokescreen to safety— to Naruto's and Ino's sides, who had decided to come anyway, even though she had told them not to.
"Run," said Sakura, her abject fear evident in her eyes.
"Who's that girl?" asked Naruto, pointing at the smokescreen.
Now that they weren't in the dark shack anymore, Sakura could finally catch a glimpse of her mysterious attacker's appearance. She was very pretty, and she had long, shoulder-length black hair. Strangely enough, she was wearing the same type of clothes that the original Sasuke had worn while he'd been staying with Orochimaru… Nope, she had no idea whose OC this was, but she wasn't finding it very funny.
!!!
Upon seeing Sakura's backup, Nanashi suddenly shut one of her eyes, and blood-tinged tears began to stream from the other, carving crimson trails down her cheek. She was looking at…
"Naruto!" Sakura screamed.
Time seemed to slow down to a crawl, and Sakura found her body to be moving on its own, pushing Naruto out of harm's way…
…and then the Amaterasu's black flames burst across her back, blacker than the darkest night and hotter than hell's hottest flame. Sakura fell to the ground in wordless agony, the flames spreading rapidly across her body…
"Water, we need water!" shouted Ino in a panic. "Right, the sea's right there! Naruto, hold her off while I go fetch some!"
"…Sakura…?" Naruto whispered.