Every civil war between Mangekyō Sharingan users ultimately evolves into a clash of Susanoo.
This is because the Susanoo represents the pinnacle of the Uchiha clan's power and the ultimate manifestation of Yin Release. By giving form to the internal craving for strength through Chakra, one achieves a perfect union of offense and defense – this is what makes the Susanoo unique.
Furthermore, because the thoughts, consciousness, and experiences of every Mangekyō Sharingan wielder differ, the abilities and appearance of their Susanoo vary wildly.
Itachi's Susanoo wields the Sword of Totsuka in its right hand; once a target is pierced, they are instantly drawn into the sake jar and sealed within a world of drunken dreams. In its left hand, it carries the Yata Mirror, which can alter its attributes based on the nature of the incoming attack, effectively reflecting all Ninjutsu and physical strikes.
Beyond these, he possesses a trump card known as Yasaka Magatama, allowing him to hurl Susanoo Chakra in the shape of giant magatama, resembling the Sharingan pattern, to trigger violent explosions.
In comparison, Sasuke's Susanoo was slightly inferior. Perhaps because he had only recently mastered it, or because he hadn't used it enough times, he lacked the seasoned proficiency of his elder brother.
For now, he could only rely on the long-range advantage of his bow and arrow, constantly maintaining distance while firing streaks of Chakra arrows coated in the black flames of Amaterasu.
Since neither side intended to hold back, the battle appeared exceptionally fierce.
This was especially true for Itachi, whose body had reached its absolute limit. Not only were his eyes streaming blood, but blood was also gushing from his mouth and nose. It looked as though he might collapse and die at any moment.
Yet, through sheer, stubborn willpower, he held on. He had to give his beloved younger brother one final lesson before he passed.
"Sasuke, do you know what your greatest weakness is?" Itachi questioned in a low voice, manipulating the Yata Mirror to deflect a Chakra arrow wreathed in black flames.
"Weakness?"
Sasuke subconsciously furrowed his brows. As an intensely proud and conceited Uchiha, he had always pursued perfection. Whether it was Taijutsu, Ninjutsu, Kenjutsu, Genjutsu, or even the incredibly difficult Sage Mode, he had mastered them all. To his mind, he had no obvious weaknesses. Even if he did, they weren't things an enemy could easily exploit.
Itachi clearly knew his brother well. He smiled and nodded. "Exactly. You may not realize it yet, but I will show you through my actions."
The moment the words left his mouth, he commanded his Susanoo to hurl a massive red magatama.
"Yasaka Magatama!"
In the next instant, the object, patterned like a Sharingan, sliced through the sky in a beautiful arc like a giant shuriken, circling behind Sasuke and detonating.
BOOM!
Crimson flames instantly engulfed Sasuke's purple giant.
However, for a Susanoo possessing near-perfect defensive capabilities, such an attack was insufficient to break the guard; it merely obscured his vision.
Sasuke, unwilling to stay put and take a beating, immediately leaped out from the surrounding inferno. But the moment he cleared the flames, he found that Itachi was already waiting for him.
"Sword of Totsuka!"
The blade, composed of pure spirit energy, bypassed the Susanoo's protection and pierced the Cursed Seal of Heaven on Sasuke's neck, completely eradicating the hidden danger Orochimaru had intended to use for his own resurrection.
With this task complete, Itachi coughed up another massive spray of crimson blood. He could no longer maintain the ultimate power of the Mangekyō Sharingan. Stumbling to the ground, he spoke in an extremely weak voice:
"Do you see, Sasuke? That is your weakness. You are too proud, too conceited, just like most of the Uchiha. Consequently, your judgments are often too hasty... or rather, not cautious enough."
"You... you removed the Cursed Seal?"
Sasuke clutched the area where his shoulder met his neck, his face turning pale. He was no fool; he understood that if the Totsuka blade had hit him instead of the seal, more than just the curse would have been sealed away forever.
"Yes. The Cursed Seal of Heaven was Orochimaru's trump card for resurrection. While it helped you refine and use natural energy more easily, the risks were far too high. Besides, now that you have obtained the Eternal Mangekyō, you no longer need it."
Having said his piece, Itachi pulled a glass vial from his robes. Before the eyes of everyone present, he plucked out his own nearly blind eyes, carefully placed them in the vial, and held it out to Sasuke.
"Here. Take them. This is the last thing I can leave you. Promise me... no matter what happens, you must live on."
"Don't worry. I won't just live on; I will revitalize the Uchiha clan."
Sasuke took the blood-stained crimson orbs with a look of profound conflict, his emotions a turbulent mess.
"Heh... in that case, I can leave in peace."
As the last word escaped his lips, the blood from Itachi's mouth and nose poured out like an opened floodgate. He fell backward onto the earth and drew his final breath.
Just as the onlookers felt a pang of regret for this young genius, a figure suddenly descended from the sky. Seizing the moment of Sasuke's distraction, the figure snatched away Itachi's body.
It was none other than Kabuto Yakushi, who had been lurking nearby for a long time. It was obvious the man intended to use the corpse for Reanimation, turning Itachi and his Mangekyō Sharingan into a powerful pawn.
"You despicable rat! How dare you?!"
Sasuke exploded in rage, roaring as he unleashed a black-flame arrow of Amaterasu.
"Almighty Push!"
To a Kabuto now possessing the Rinnegan, ordinary attacks were useless. By simply manipulating gravitational forces, he easily diverted the arrow from its path.
As a spy accustomed to caution, Kabuto did not linger while surrounded by core members of the Akatsuki. Having secured what he wanted, he used the Body Flicker Technique and vanished without a trace.
"Dammit! Kabuto Yakushi! I swear, no matter where you run, I will kill you!"
Sasuke let out a cry of powerless fury. It wasn't that he didn't want to give chase, but the battle with his brother had drained too much Chakra. Furthermore, both his Mangekyō Sharingan were throbbing with pain from overuse. Even if he caught up, he might not be able to overcome a Kabuto who possessed the Rinnegan and Sage Mode.
As for the others, they mostly knew this was a part of Allen's script, so they had no intention of interfering.
And so, the civil war between the Uchiha brothers finally came to a close. Itachi had successfully passed his eyes to his beloved brother, while simultaneously setting the stage for the upcoming Reanimation.