Having completed his revenge, Sasuke parted ways with Kabuto and set out alone, hoping to find meaning in his existence.
He drifted across the sea in a small boat.
Along the way, a resonance with Madara awakened the Rinnegan in his right eye, and he passed out.
When he awoke, he discovered that the currents had washed him ashore.
He partially covered his right eye. Ripples and tomoe had combined into a new pattern, unmistakably different from anything a Sharingan should possess.
"It wasn't an illusion. I really did awaken the Rinnegan..."
"If Kabuto knew, he'd probably jump for joy."
"Where is Kabuto now? What's he doing?"
He shook his head, casting aside those idle thoughts.
After accepting his situation, he assessed his surroundings.
"This season, this temperature... somewhere subtropical. The coast of the Land of Lightning? Then I must have been drifting for at least half a month..."
A lingering fear crept over him. He had been unconscious at sea for so long that even the slightest mishap could have cost him his life.
"Why did I suddenly awaken the Rinnegan? Could it... have something to do with those images that appeared in my mind earlier? What were they?"
"And why was I unconscious for so long?"
Those images had come from the resonance with Madara's left eye.
Kakashi and Obito had experienced something similar years ago.
Sasuke had remained unconscious for so long because he had not yet reached the threshold for awakening the eye, but the resonance had forced it to awaken anyway.
More specifically, when his eye awakened, the hashirama cells in his body had gathered around his right eye in vast quantities, leaving him utterly depleted!
Sasuke took a Military Rations Pill from his supply Ninja Tool Pouch, placed it in his mouth, and chewed.