It's worth mentioning that Senju Mori was born with both Water and Earth chakra natures.
As for his Wind chakra nature transformation, that was something he gradually mastered through years of combat and training after being promoted to jōnin.
For most shinobi, being born with two chakra natures is already rare enough — but for a member of the Senju Clan, possessing Water and Earth specifically carries a special significance.
This discovery had once made several of the Senju clan elders quite excited.
Even though Senju Hashirama, the First Hokage, was a man of gentle temperament, his very existence as the "God of Shinobi" held immense symbolic weight for the entire clan.
However…
Despite having Senju blood flowing through his veins and being born with Water and Earth chakra, Mori was still unable to learn the Wood Release — the legendary kekkei genkai said to "bring peace to a chaotic world."
Although this disappointed the old men of the clan, Mori himself didn't care much.
He knew that even if he could somehow perform the dual Water–Earth transformation and successfully manifest Wood Release, the power he could muster would be no greater than that of Yamato in the future.
Without the Yang power of the Sage of Six Paths, and without Hashirama's own Sage Body and Sage Mode, it was impossible to reproduce that godlike, world-shaking level of Wood Release.
Rather than waste time and energy chasing a jutsu that held more symbolic than practical value, he preferred to devote himself to learning techniques that were truly useful in battle.
With his mastery of kenjutsu and taijutsu, and his expert command of the Water, Earth, and Wind chakra transformations, Senju Mori's current strength was not to be underestimated.
...
"Earth Release: Flowing Earth River!"
Coordinating with his earlier large-scale Water Release, Mori quickly transformed the wide stretch of solid ground within sight into thick mud. The once grassy field turned into a swamp capable of swallowing men whole.
Even a monster like Kinkaku, half-transformed into a tailed beast, couldn't fully defy the laws of physics as long as he couldn't fly.
However, Kinkaku was barely hindered. He dropped to all fours like a true beast, his limbs coated in claw-shaped tailed-beast chakra, and charged toward Mori with terrifying speed.
Clearly, the sneak attack from earlier had infuriated him completely.
Mori's face remained calm. He dropped to one knee and pressed his left hand against the ground.
At once, the terrain beneath Kinkaku seemed to come alive.
The earth around him rapidly alternated between muddy swamp and solid rock, throwing his balance into chaos. Even with four limbs on the ground, the shifting, uneven footing made it nearly impossible to move smoothly.
At the same time, Mori formed one-handed seals with his free right hand.
Immediately, countless sharp earthen spears erupted from the ground around Kinkaku, thrusting upward toward his chin, abdomen, and even the base of his thighs—merciless, underhanded strikes aimed at his vital points.
The barrage was so ruthless that even the beast-like Kinkaku had to retreat. Kicking off with his hind legs, he leapt onto a nearby massive tree trunk, then used it as a springboard to pounce through the air once more.
Mori instantly lifted his left hand from the ground and rose to his feet. While his fingers flashed through a new sequence of seals, his cheeks swelled with gathered chakra.
"Wind Release: Dust Storm!"
In an instant, all the moisture from the swampy terrain seemed to vanish, leaving the ground cracked and bone-dry. The fierce wind Mori exhaled whipped up blinding clouds of dust and sand, shrouding everything in sight in a hazy storm.
Of course, such a simple visual trick couldn't truly stop Kinkaku's ferocity.
Within a second, his massive figure burst through the dust storm, his claws driving straight through Mori's chest—piercing the heart cleanly.
Yet… not a single drop of blood flowed from "Senju Mori's" body.
The humanoid figure suddenly collapsed into a pool of thick mud, then hardened again in less than a second—momentarily binding Kinkaku's half–tailed beast–form body in place.
In that same instant, Senju Tobirama's figure appeared directly behind him.
The high-velocity water blade flowing around Tobirama's right hand slid effortlessly into Kinkaku's body, and as Tobirama's arm swept across, the blade widened the wound in a single, fluid motion, cutting through flesh and chakra alike.
The unbearable pain triggered a violent reaction. Countless bubbles erupted from the surface of Kinkaku's tailed-beast cloak, and in the blink of an eye its crimson hue deepened into a dark, blood-red tone.
Three additional tails tore their way out of his back one after another.
An unprecedentedly dense surge of tailed-beast chakra engulfed Kinkaku completely, blotting out his human features. His eyes and gaping jaws now shone only with white chakra light, while a pale exoskeleton began to crystallize over his body.
Yes—a monster.
Kinkaku had become a true beast. He no longer needed to perform hand seals or techniques; the chakra itself burst out of him wildly, uncontrolled, like a living storm.
Having struck successfully, Tobirama vanished from sight without hesitation. The sudden explosion of power and chakra allowed Kinkaku to rip free from Senju Mori's earth-style restraints. He turned and swiped with one massive claw, carving a deep trench straight through the forest.
To Mori's astonishment, Kinkaku still retained some shred of awareness. Those twin, hollow white fox-pupiled eyes suddenly locked onto him—
He had sensed where Mori was hiding.
Boom!
The ground exploded as Kinkaku vanished from sight.
Mori had no time to dodge; he hastily hardened his skin with Earth Release, but the next moment he was struck full-force, sent hurtling through the air like a rag doll.
If not for the deep claw marks left on the ground, Mori might have thought Kinkaku had somehow learned Tobirama's Flying Thunder God Technique mid-battle.
Thanks to the Earth Hardening Technique, Mori survived — though embedded deep in a rock wall, he wasn't fatally injured.
Still, the moment he opened his mouth, a wave of nausea hit him harDd.
He knew the feeling well — his organs had been compressed.
Touching his side, he could feel that two ribs were broken.
If an ordinary shinobi without hardening had taken that hit, they'd have been smashed into a bloody mist on the spot.
Whoosh—!
Another enormous chakra tail whipped through the air with a deafening crack.
Already anticipating the attack, Mori instantly melted into the earth, dodging it while using Earth Release to burrow rapidly toward Tobirama's position.
Tobirama's face had grown noticeably paler, yet his expression remained unchanged; his voice was as calm and composed as ever.
"Don't worry," he said evenly. "He's not a true jinchūriki. His chakra and his body's tolerance both have limits. He won't be able to maintain this state for long."
"Even now, as his chakra rages, the last of his consciousness is fading fast."
"Once he completely loses himself and the transformation breaks, he'll lose most of his fighting power."
"Then we should—"
Mori's face twisted into a strange look—his eyes gleaming with an unmistakable eagerness to test something.