The problem with the Rinnegan was beyond everyone's understanding. Back then, older Sasuke had been too badly injured, so Hui could only patch up his superficial wounds.
But Hui did not have a Rinnegan himself and had no idea how to repair one. Orochimaru proposed a solution—injecting a bloodline-purifying serum made with Kaguya as the raw material.
As for where the material came from, it could only be attributed to how easy their ancestor was to fool. With just the excuse of a physical examination, she obediently let Tsunade examine her from head to toe.
Whenever those two had free time lately, they had been researching the secrets of the chakra fruit. Who would have thought that older Sasuke would insist on courting death by swapping positions with four-dimensional matter, leaving his soul...