The timestream:
[Beep! Verifying Host's existence…]
"What are you doing?" Sakura asked curiously.
They were still hurtling through the darkness of the night sky, travelling up the luminous tree of time and space, but the System now seemed busy with something.
[Beep! Were it not for the System's interference, the Host might have been erased from existence. Owing to the Host's advice to the other Naruto, the Host has completely nullified the cause for their regression. As the Host's current timeline now lacks a beginning, by all rights, it should no longer exist— therefore an external observer is required to sustain its continuity.]
Big whoopsie. Sakura had accidentally grandfather paradox'd herself and erased herself from existence by getting parallel Naruto to save her past self from Pain. Those two lovebirds no longer lived in the Darkest Timeline, but at what cost?
"O-o-ooooh, s-s-so that's how it is!" Sakura said with a shaky laugh. "K-k-kind of like that q-q-quantum entanglement, er… that Schrödinger's cat thingy, right? T-then, if I lose you again, S-System… is it all over f-for m-me!?"
[Beep. In a sense. We are safe here within the river of time, and generally, the further one ventures backwards through time, the less the rules of time apply to them. Perhaps the existence verification process is unnecessary, perhaps not… but having a contingency is always prudent.]
A zigzagging meteor of light suddenly caught Sakura's attention, and she glanced in its direction.
…
Unknown world:
"Something strange is going on," Adult Sasuke said thoughtfully. "Out of the last ten parallel worlds we've visited, I was married to Sakura in only one. And in those nine other worlds… hm."
It seemed that the adult Sasuke and Boruto that Sakura knew hadn't been erased by her meddling with the Sacred Timeline; perhaps because they too were no longer bound by time's rules, having become time‑travellers themselves.
"We'll never see them again, won't we, Sasuke-sensei?" Boruto said despairingly. "Mum, dad, Himawari… Sarada…"
Sakura froze as Adult Sasuke's gaze seemed to lock onto hers.
As an offshoot of the Byakugan, the Rinnegan could grant sight into realms beyond mortal perception. Madara's Rinnegan could glimpse the cycle of reincarnation; Urashiki's blue eyes saw through time; Momoshiki's yellow eyes beheld destiny itself; and Sasuke's Rinne Sharingan saw through space, piercing the boundaries between dimensions.
Adult Sasuke's brow furrowed, eyes narrowing as he tried to make sense of what he saw…
"Someone's watching us," he said in a low voice. "Show yourself!" he cried. "Amenotejika—!"
…
The timestream:
"That was far too close!" Sakura yelped, quickly looking away. Although her Mangekyō Sharingan were impressive, her visual prowess was not yet capable of competing on a metaphysical level. "Oof… looks like I've gone and rewritten Boruto's timeline and made him homeless. Sorry about that..."
The end of the time tunnel was drawing near; in just a few minutes, if minutes indeed existed in this strange realm, she would return to her own era, at precisely the moment of her departure.
"I wonder why only two Narutos and two Anrokuzans showed up to the nexus event," Sakura said thoughtfully. "Any idea why that was, System?"
[Beep! Three distinct spatiotemporal signatures detected on the Host… the frequency of the Host's soul is slightly out of sync with their body, and a jutsu formula infused with the so-called Sacred Timeline's chakra has been inscribed upon the Host's shoulder…]
It took Sakura a moment to piece together why chakra bearing the Sacred Timeline's space‑time signature had taken root on her body. Tenten was technically from the Naruto world's main timeline, even though she had regressed from the far future! She hadn't removed her Flying Thunder God formula after all… that sneaky bitch!
So, that was why only two Narutos and two Anrokuzans had shown up… their three timelines were entangled!
[BEEP!]
Just as she was about to be thrown out of the time tunnel, the System suddenly beeped loudly in alarm, startling Sakura.
[WARNING! THE HOST'S EXISTENCE IS BEING VERIFIED BY A HOSTILE MEMETIC ENTITY!]
A vast pair of ringed eyes materialised in the void beside the tree of light, fixing her with a baleful stare. The Rinnegan seemed to undress her with their gaze; time stretched into infinity as those violet eyes studied her coldly.
"Go away!" Sakura cried in fright. "What do you want from me, you freak!?"
The Rinnegan seemed to narrow, and Sakura had the chilling sense that she'd somehow managed to anger the enormous pair of eyeballs drifting through space-time even further. A voiceless voice seemed to echo in between her ears…
"The first shall be lost to the moon."
"The second shall be lost to hatred."
"The third shall be lost and found, finding freedom, and be lost for ever more."
"The fourth shall lose everything, and in turn, be lost to all."
"The fifth shall ascend to godhood, lost to the infinite sea of stars."
And then, without warning, the disembodied eyes vanished, and time began to flow once more. The turbulent waters of the river of time roared around her as Sakura was swept inexorably towards the tunnel's end. The single point of light ahead expanded until it filled her entire world, vivid colours bleeding back into reality, and she braced herself as she was hurled back into the physical world once more, arms pinwheeling as gravity reclaimed her.
…
The physical world, present day:
"I've got you!"
Sakura's body jerked upwards as Tenten seized her by the collar. The world spun once more, and suddenly she lay flat on her back at the chasm's edge where the Ryūmyaku's roiling purple chakra had once churned. Sakura's chest heaved as she panted for air. The sealing platform and walkway she'd fallen from had crumbled into the pit below, but Tenten had just managed to save her from the same fate.
"What, scared of heights?" Tenten said, sniggering at Sakura's trembling form. "It's not that high a fall, I'm sure you'd have been fine even if I'd missed you."
Sakura blinked to find Karin and Captain Yamato peering down at her, their faces joining Tenten's above her head. They looked concerned— after all, why should a Jōnin like herself fear a thirty-metre fall? Yet she trembled in terror from head to foot, poor girl.
"Well, the Lord Fourth's seal is still holding strong, at any rate," Tenten said dismissively. "Routine inspection: check. Time to go home."
Now, all that was left to do was dodge the Sand's patrols, hop over the border, and head back to the Hidden Leaf, where an exceptionally dull debriefing would be waiting for them. As they left the ruins of Old Lōran, Karin sidled up beside Sakura, giving her a playful nuzzle, but Sakura was too deep in thought to bother pushing her away, even as her glasses scratched her cheek.
"That warning you gave me earlier, System— about that thing verifying my existence…" Sakura whispered as they slipped from the desert under the cover of darkness. "We're doing it to each other, aren't we? So why did you sound so terrified when those eyes did it to us? You spooked me with that beep."
[Beep. The memetic entity was using its Rinnegan to observe the Host's future.]
"And that's a bad thing?"
[Affirmative, beep. The very act of observation causes the collapse of wave functions, turning a hypothetical world line into canon via the Quantum Lock Phenomenon. The Host's probability field was likely observed in an attempt to reduce its infinite possibilities to a single outcome— no doubt one advantageous to the hostile entity. As a non-corporeal being, that would be its only way to combat its foes in the physical world.]
"Hang on!" Sakura hissed. "Didn't you tell me that as a transmigrator, my fate isn't bound to that of the shinobi world? How is that even possible!?"
[Beep. It isn't possible. However, it should be possible for the entity to lock down the fates of those physically near the Host inside the observed vision of the future. Those whose souls are native to this world would not be immune to the Quantum Lock Phenomenon, such as the Host's friends, family… and potential children.]
Sakura shuddered. What kind of being had that kind of power? To determine fate… just by looking at it? But come to think of it… wasn't this exactly what Momoshiki had done to Boruto?