(p.s. no exact method explained as to how Eye Transplantation for Eternal Mangekyou Sharingan is done, the method below is pure bullshit, but it is what it is, so just accept it.)
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To seal Daikokuten into Uchiha Ryota's right eye with the help of the Transcription Seal, Kuroto must first transplant Uchiha Hideki's Mangekyou Sharingan.
Kuroto would obviously not risk removing the Tenseigan from his main body, and since there are no other clones aside from Suijin Clone currently so Suijin Clone is the only option, he has to use for the fusion process of the two Mangekyou Sharingan pairs.
Using the surgical instruments, Kuroto carefully removed the semi-finished Byakugan from the eye sockets of Suijin Clone.
In the future when he is going to use the Suijin clone for the ritual to achieve Kekkei Mora through the Chimera technique, there must be no defects in his main body or any of the clones, even the eyes in the eye sockets of the clone will also be the same semi-finished Byakugan, therefore, this pair of semi-finished Byakugan must not be lost or destroyed, otherwise, the whole Clone he has cultivated up to now will become useless.
Therefore, Kuroto carefully sealed the semi-finished Byakugan in the preservation solution and then transplanted Hideki's Mangekyou Sharingan in the eye sockets of Suijin Clone.
Next, Kuroto brought out Uchiha Ryota's body that was in a temporary stasis and reinstalled his Mangekyou Sharingan into his eye sockets.
With that done, the initial preparations were completed, and the next thing he has to do is to use the Transcription Seal - 2.
For this purpose, Kuroto again used the Tenseigan Soul Descend to transfer his soul into the Suijin Clone.
Coming to the test bench where Uchiha Ryota laid unconscious, Suijin activated the Mangekyou Sharingan.
Then after completing all the preparation, Suijin used his index finger and gently tapped Uchiha Ryota's forehead.
As soon as the action was performed the Transcription Seal activated and Hideki's Mangekyou Technique Daikokuten was sealed into Ryota's right Mangekyou Sharingan.
The moment the Transcription Seal was completed, Suijin felt a sudden weakness.
"Hah… Hah… Hah…
Leaning against the wall, Suijin panted for a while to ease his breathing.
"Using this clone body to cast the Transcription Seal is quite difficult!"
When Kuroto used his main body to cast the Transcription Seal from Tenseigan, he only felt a momentary weakness, but this time, he used Suijin Clone to cast the Transcription Seal from the transplanted Mangekyou Sharingan, so he had to bear more pressure.
"Fortunately, the Transcription Seal only needed to be used once otherwise this clone would have been damaged due to bearing excessive pressure!" – Suijin muttered while shaking his head slightly.
After recovering, he tried to perceive the Transcription Seal, and as he expected, there was nothing he could perceive from Ryota's right eye, so changing his approach, Suijin tried to check his eyes and found that Daikokuten was exhausted from his eyes.
The reason why it was lost was that he transferred it completely.
Confirming his guess, Suijin muttered, "I just hope that my guess is right."
The reason for going to so much trouble is to ensure that the Eternal Mangekyou Sharingan that will be obtained after the fusion of two pairs of Mangekyou Sharingan can have Daikokuten in the right eye and Yaoyorozu in the left eye.
However, this will only happen if his assumption that 'Transcription Seal will have some influence' is correct, otherwise, the Eternal Mangekyou Sharingan will not have Daikokuten, instead, it will have Okuninoshi, which is the original dojutsu that Ryota awakened.
After completing the second step, now only the final step is to implant both Mangekyou Sharingan so as to initiate their fusion into the Eternal Mangekyou Sharingan pair.
To do this, he used the Tenseigan Soul Descend and transferred back his soul to the main body.
The next was to place the Suijin Clone on the Test Bench and remove Hideki's Mangekyou Sharingan pair from the eye sockets and replaced the semi-finished Byakugan pair back to the eye sockets it belonged to.
With that done, Kuroto took the removed Mangekyou Sharingan pair and walked to the other test bench on which Uchiha Ryota was lying.
The reason Kuroto did not kill Hideki and Ryota was that he knew that later on, he would need them to fuse the two pairs of Mangekyou Sharingan into the Eternal Mangekyou Sharingan.
Because the fusion process requires that either one of the owners of the Mangekyou acts as the host body to initiate and carry out the fusion process, the reason is obviously to provide a continuous and stable chakra supply.
As for who to choose as host, this decision entirely depends on Kuroto.
And since Kuroto has chosen Ryota's Mangekyou Sharingan pair as the base because of the Mangekyou Techniques, therefore, Kuroto is obviously going to choose Ryota to be the host.
Coming to the side of the Test Bench on which Ryota was kept, Kuroto reconfirmed that all the seals that he has cast on him were working, as well as all his indicators were normal, and since everything was right, exactly as it should be, Kuroto took a deep breath and started the final step to the fusion.
The fusion of two pairs of Mangekyou Sharingan into the Eternal Mangekyou Sharingan happens when one pair completely swallows the visual prowess of the other pair.
Therefore, the surgery for fusion is actually transplanting (not the correct term to what is referred to here) Hideki's Mangekyou Sharingan into Ryota's eye sockets, who already has his pair of Mangekyou Sharingan in his eye sockets.
In short, Ryota's eye sockets will hold two pairs of Mangekyou Sharingan one-over-another, at the same time.
And this is also the reason why the pattern of Eternal Mangekyou Sharingan is a combination of both pairs of Mangekyou Sharingan.
The space in the eye sockets is obviously limited, and the area that connects the optic nerve is extremely delicate, so it is actually very difficult to accommodate two eyeballs in one eye orbit at the same time.
Even a small mistake will not only damage the orbits, especially the nerves, it will also damage the Mangekyou Sharingan pair, which will directly lead to the failure of the fusion process, so even Kuroto who already has rich experience in transplanting eyeballs will have to be extremely careful when carrying out this overly complicated operation.
After opening Ryota's eyelids with the help of the surgical instruments, Kuroto started the operation to transplant the left eye.
Human eye sockets are surrounded by soft tissues, and the eyeballs are also made up of soft tissues, as such, it is not impossible to squeeze it to accommodate two eyeballs within the eye socket. What's important is to ensure that neither the eye orbits nor the eyeballs are damaged during the transplantation process.
The difficulty of this operation is so high that it took Kuroto one whole hour to transplant the left eye alone.
After the left eye was transplanted, Kuroto breathed in relief and wiped away the sweat from his forehead.
The transplantation of the left eye was successful without any rejection.
After drinking some water and a five-minute break, Kuroto again started the transplantation operation, this time for the right eye.
Similar to the left eye, the right eye was also transplanted smoothly without any rejection.
With this, the transplantation operation is completed, and Kuroto wrapped Ryota's eyes with an antibacterial cloth that will also keep the eyes moist.
Kuroto does not know how long it took Uchiha Madara to fuse the two Mangekyou Sharingan into Eternal Mangekyou Sharingan pair, but he does remember it very vaguely that it took Sasuke a few days to completely merge the two Mangekyou Sharingan pair into the Eternal Mangekyou Sharingan.
And considering the fact that Ryota is and will stay unconscious, the time required for fusion may in fact be longer than Sasuke.
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Read up to Chapter - 501 on P.a.t.r.o.e.n. Page: p-atreon.com/Milta_translations