In a mysterious pocket dimension of the Ninja World, there exists a forbidden land of towering bone forests.
A primitive and ancient sovereign has entrenched itself upon this land, transforming it over a millennium into a paradise belonging to it alone.
Acid continuously seeps into the earth, gradually drying, then reappearing at the tips of the densely pitted bone forests, forming massive cascading waterfalls.
Breezes laden with acid rise, carrying thick acidic mists, sweeping past the porous bone forests, emitting shrill, mournful howls.
Amidst these desolate cries, Sakura was summoned to this death-laden forbidden land through the Reverse Summoning Technique.
"Eek! Sakura-chan, are you alright?!"
Seeing the girl inhale the first breath of air here, Katsuyu's clone looked at her anxiously.
"Huh?" Sakura was puzzled. "Why should there be anything wrong?"
"Allow me to explain!"
Accompanied by a gentle voice, the ground beneath Sakura's feet began to undulate.
The Katsuyu that had just summoned her earlier seamlessly merged into the ground, rising together with a colossal entity.
Its head alone was the size of an entire Konoha Village.
"This sacred land is saturated with my acid and Chakra. The Natural Energy contained within, even if passively inhaled, carries the risk of transforming you into a slug."
The Katsuyu Sage gently turned its head, its frontal antennae rotating to look at Sakura's position. "It's not like there haven't been other contractors who were transformed the moment they stepped into Shikkotsu Forest~ But don't worry. It mostly depends on personal resistance."
'Great. It seemed the future training would be fraught with hardships.'
'And those difficulties would likely come from this Katsuyu Sage, who spoke the most terrifying outcomes in the gentlest voice.'
"Enough small talk, Katsuyu Sage. I'm here to train in Sage Mode!" Sakura got straight to the point.
Currently, the Akatsuki had gathered seven Tailed Beasts. The Ninja World was about to undergo a major version update.
By then, ordinary Ninjutsu would be mere after-dinner desserts, adding to the amusement of the Otsutsuki Clan with their higher Chakra privileges.
The methods to counter them were simple: first, if you can't beat them, join them, or second, a path different from theirs… Sage Mode!
"Sage Mode?" Katsuyu was momentarily taken aback. "That... requires training?"
"..."
Sakura's expression subtly changed, a vein throbbing on her forehead, clearly struggling to hold back her temper, but she ultimately didn't lose it.
"Ara ara, sorry, sorry! I forgot you're still considered human!"
Sensing the change in Sakura's mood, Katsuyu's gentle voice sounded somewhat guilty.
'Hmm, I can't tease her any more. This child will get angry.'
"Sigh, I understand you enjoy interacting with humans." Sakura sighed softly. "But with the imminent revival of the Ten-Tails, there isn't much time left for leisurely play!"
"Gee, Sakura-chan is just too serious."
A slug split, about two people tall, squeezed out from the part of Katsuyu's body where Sakura stood, carrying her and walking deeper into Shikkotsu Forest.
"As a long-lived species, you need enough humor and leisure to resist time's erosion~"
The consciousness within the now-smaller slug remained vast. It seemed its physical forms might be akin to a hive mind, allowing it to freely connect with any body.
With the main body acting as a massive relay station, no wonder Katsuyu could transmit a contractor's healing Chakra flawlessly to every clone across distances.
"You noticed?"
Hiding it from Katsuyu was pointless. Sakura candidly admitted.
"Mhm. And judging from your life force, you can now live much longer than when you first signed the contract!" After a careful examination with its rotating eyes, the Katsuyu Sage reached its conclusion.
"You can tell that... does it have to do with Shikkotsu Forest's Sage Mode?"
Though Sakura phrased it as a question, her tone was very certain.
"Ah, what a sharp child!"
Katsuyu's voice became even gentler, seeming pleased with Sakura's perceptiveness.
'Hmm, clever, strong, long-lived, and can even take me to other worlds to play. This contractor was the best one I had found. Surely, in the days to come, she would accompany me through my long life!'
Pulling Sakura along, they stopped before a massive lake.
The surroundings were still scarred bone forests and wastelands. The increasingly dense mist made the scenery appear hazy.
Katsuyu set Sakura down, nudging her back with its head.
"Go ahead. That lake ahead is where one trains in Shikkotsu Forest's Sage Mode. But I suggest you don't immerse your whole body in the water all at once."
Given Katsuyu's instructions and her prior caution towards this sly creature, Sakura didn't choose to use too much of the lake water at once. She only lightly touched the surface with the back of her hand.
Sizzle, sizzle, sizzle!
The moment her skin made contact with the lake water, an intense burning sensation spread from her hand.
Natural energy fiercely corroded Sakura's skin. Even her [Steel Skin] couldn't withstand corrosion of this level.
No, rather than corrosion, it felt more like a... regression at the cellular level?
"What on earth is going on?"
With a heart full of questions, Sakura turned her gaze to the Katsuyu Sage behind her.
Simultaneously, she formed a Chakra Scalpel with her right hand, slicing off the transformed flesh, then healing it with the Mystical Palm Technique.
"This is Shikkotsu Forest's Sage Mode… the Celestial Evolution Technique!"
Katsuyu slithered to Sakura's side, enveloping her, using its own energy to accelerate her regeneration.
Natural Energy encompasses all creation. Combined with an individual's will, it gives birth to a myriad of abilities.
The Gift of the Hermit Group of Fire Temple, Orochimaru's Cursed Seal, Jugo's Flesh Mutation...
The Sage Modes of the three great Holy Lands also have their own characteristics. Their cores lie in the different original intentions of their creators.
The toads of Mount Myoboku sought prophecy, desiring to control the future. Hence, their Sage Mode primarily greatly enhances the user's sensory abilities while granting energy manipulation powers.
The serpents of Ryuchi Cave desired to become dragons, pursuing transformation and dominion over natural phenomena.
Thus, their Sage Mode mainly manifests as the user's serpentine/draconic transformation, along with the ability to manipulate inorganic matter.
Finally, the Katsuyu Sage. Its wish was simple and straightforward: it just wanted to live.
Therefore, its Sage Mode possesses the ability to manipulate organic matter. This pool of acidic liquid, gathering natural energy, is the most basic application of its Sage Mode… a regression at the cellular level, reverting it to a younger state!
"Isn't this immortality?!" Glimpsing the immense value hidden beneath this technique, Sakura couldn't help but exclaim. "Of the three Holy Lands, you're actually the one hiding the deepest?"
"Well, that's a more basic application. Actually, you can also perform living organism modification, edit genes, and such~"
Praising Katsuyu made it shyly wriggle its body, excitedly raising its antennae, looking like it wanted more praise.
"But how do I train in this technique?" Sakura glanced at the acidic liquid before her. Setting aside withstanding the strong acid corrosion upon immersion, if she accidentally got hit by the residual technique and was reverted to a fertilized egg, the joke would be on her.
"Oh, that's simple. You just need to continuously divide and expel the reverted body cells!" Katsuyu's tone was light, as if discussing where to eat today.
"How can a human do that?" Sakura clutched her forehead in pain.
"Eh? Can't you? I saw Tsunade-chan could do it~"
Katsuyu extended a tendril, tapping Sakura's forehead. "That technique that stores Chakra here, then releases it all at once to expel damaged parts of the body."
"Wait, are you saying... the Yin Seal - Strength of a Hundred Seal?"