The night of Naruto's birth was the moment Aizen had long anticipated. While Obito (the masked man) believed he was the one in control by summoning the Nine-Tails to destroy Konoha, Aizen stood atop the "Secret Observation Tower," watching the chaos with divine coldness.
To Aizen, the Kyuubi wasn't just a monster of chakra; it was a massive "energy vessel" lacking order.
The Forbidden Ritual: The Soul Fragment
While Minato was occupied battling Obito, Aizen exploited the moment of weakness in the seal. He didn't appear physically; instead, he sent an advanced "Spirit Clone" through the spatial void created by the Hogyoku.
The spiritual Aizen stood before the giant fox, which was roaring and tearing through the village. With a calm motion, Aizen reached out, and a violet glowing crystalline piece emerged from his chest—a fragment of his soul fused with the power of the Hogyoku.
"Do not scream, beast," Aizen whispered, his voice echoing directly inside the Kyuubi's mind. "I am not trying to imprison you like the humans... I am giving you a 'consciousness' that transcends instinct. I shall be the voice that guides you from within."
With a single thrust, Aizen planted his soul fragment into the Kyuubi's "Chakra Core." In that instant, the fox's red eyes flashed violet for a fleeting second, and its entire being shuddered. No one sensed this change; even Minato, preparing to seal the beast, believed the disturbance was due to the weakness of the old seal.
The Long-Term Plan
When Minato sealed half of the Kyuubi inside Naruto, he didn't know he was also sealing "Aizen's seed."
"Now," Aizen thought as he returned to his physical body in silence, "Naruto will not just be a Jinchuriki... he will be the incubator for my soul. As the boy grows, my power will grow within him, until the time comes when the fox awakens to find it serves not hatred, but Sosuke Aizen."
Aizen looked at the Fourth Hokage sacrificing his life and felt no remorse.
"Farewell, Minato," Aizen said as he vanished into the shadows. "You died to protect a village... while I have been reborn to rule a universe."