This will be a very short but very vital theory:
In conjunction with my theory:
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Orochimaru did not use the Shiki Fuujin Kai (Reaper Death Unseal) before because he couldn't
Because Orochimaru needed the Uchiha Sharingan to open the Naka Shrine Tablet's Room and have someone (his lackeys) put the Shinigami Mask in front of the Uchiha Tablet
Take a look at this:
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Suigetsu said "So you need a jutsu to open that door"
Notice how the Marble door has the image of the Sharingan in it instead of the usual Uchiha Crest Symbol?
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And did you notice Sasuke's Sharingan was activated after opening the marble door?
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it is because: The Sharingan (its' jutsu) was needed to open the door and enter the room where the Uchiha Tablet is.
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Secret Meeting Place
It makes perfect sense that only the Sharingan could open the door to the secret room, it's because it was also used for secret meetings of the Uchihas, what better way to protect yourself from spies when you're talking about sensitive stuff than to shut yourself in a room where only Uchiha Clan members could open.
And Sasuke already awakened the initial stage of his Sharingan during the massacre when he broke free from Itachi's genjutsu that is why he was able to enter the Shrine as a kid
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And Kabuto did not say he or Orochimaru had access to the Secret Room or the Tablet and ready/study its' contents, they merely hypothesized its' contents (they knew its' existence but couldn't access it) based on the data from their Experiments (Probably Hashirama cells, etc.) So they couldn't have possibly entered the sealed secret room of the Uchiha Tablet.
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So what was important downstairs that was needed in order to perform the Reaper Death Unseal? what was needed to activate the Shinigami Mask?
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The Answer is this:
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The Uchiha Tablet
- The Shinigami Mask is used/activated with the Naka Shrine Tablet to perform the Reaper Death Unseal.
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So this is why Orochimaru didn't have someone else use the technique for him (Sound Ninjas) to retrieve his arms earlier, it's because he couldn't open the door to the secret room
Orochimaru researched on the Reaper Death Unseal immediately after losing his arms.
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So Orochimaru found the scroll relatively early from the time he lost his arms.
That is why Orochimaru didn't use the info. on the scroll years and years ago:
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It was because he couldn't he needed the Sharingan to open the door, have someone else wear the Shinigami Mask for him and activate it on the Naka Shrine Tablet to successfully perform The Reaper Death Unseal:
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He probably obtained the scroll at an earlier point within those three years after losing his arms to RDS, but he still couldn't use it.
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He needed Sasuke's Sharingan, but he couldn't wait for the Sound Four to bring him back to his hideout because he was dying from the excruciating pain of his arm's necrosis
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So he had to possess another body (thus locking his capability to transfer to another body again for three more years)
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Orochimaru found the scroll after losing his arms, but he still couldn't use the information in it because his body transfer technique was locked for another three years, so he tucked it neatly in his secret room:
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The spiderwebs and the accumulation of dust in the room indicates the scroll had been there for years (3 years+).
He needed the Sharingan to open the room where the Uchiha Tablet is, the catalyst of the Reaper Death Unseal Jutsu:
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Orochimaru needed the Sharingan to open the Naka Shrine sealed door where the Tablet was, he needed someone other than himself to put on the Shinigami Mask in FRONT of the Tablet to activated The Reaper Death Unseal (Sound ninjas willing to sacrifice their lives for Oro, no intricate hand seals are needed, they only need to put the Shinigami Mask on in front of the Uchiha Tablet in order to activate it.)
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That is why Orochimaru became even more interested in wanting to possess Sasuke's Body (aside from his initial plans of kidnapping him to obtain the Sharingan to learn all the jutsus). He needed Sasuke's Sharingan to open the Naka Shrine Room of where the Uchiha Tablet was because the Tablet WAS NEEDED to perform the Reaper Death Unseal to retrieve his arms. But he suffered a setback because Sasuke put him down before he was able to possess his body.
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[Orochimaru intended to possess Sasuke's body to open the door, but he probably intended another person to perform the RDS Unseal either Kabuto or some other Sound Ninja to wear the mask in front of the tablet to perform Reaper Death Unseal and die, since many of Oro's followers were willing to die for him, none the less Oro needed Sasuke's body and Sharingan to open the door.]
Orochimaru didn't force Sasuke before to open the door to the Naka Tablet Room because Orochimaru knows he couldn't FORCE Sasuke to use his Sharingan and open the sealed door.
a.) Orochimaru was a non-Uchiha so Sasuke would not allow such a disgraceful character like Oro to defile the sanctity of his clan's shrine
b.) Sasuke knows full well of Orochimaru's true intentions (total possession of his body) if Sasuke helped Orochimaru retrieve his arms then Orochimaru would be in full power and would have easily taken over Sasuke's body thus depriving Sasuke of his vengeance on Itachi.
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Orochimaru needed to wait three years (after performing his body transfer technique, since three years was the time limit before he could possess a new body again) to possess Sasuke in order to use Sasuke's Sharingan to open the door to the room of the Uchiha Tablet.
So that means The Naka Shrine Tablet was the catalyst for the Reaper Death Unseal.
In conjunction with my theory:
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(Gist: Rikudou Sennin owned the Naka Tablet and it was used for the Uzumaki RDS Unseal meaning both are connected to Rikudou Sennin)
UPDATE 1
Something to ponder about:
We know that after the Tsunade Retrieval Arc, the Sasuke Retrieval Arc followed.
- Many assumed that the only reason Orochimaru wanted to possess Sasuke's body at that time (Sasuke retrieval arc) was because he wanted the Sharingan's powers to learn all the jutsus, this is false.
- Because even if Orochimaru was successful in obtaining Sasuke's body during that time, his arms' soul would still be sealed. so Orochimaru still needed to retrieve the soul part of his arms.
- So the true reason for Orochimaru wanting Sasuke's body immediately during that time was: He needed Sasuke's body to use the Sharingan to open the Naka Shrine to access the Tablet, have a Sound Ninja sacrifice his life performing the Shiki Fuujin Release, and retrieve the souls of his arms while preserving/not killing his possessed body of Sasuke at the same time.
- The only problem was he became impatient and unable to endure the pain from the necrosis of his arms, thus needing to perform the body transfer before Sasuke arrived at his lair, thus locking his technique for three more years.
- After three years when he was ready to possess Sasuke's body, Sasuke unexpectedly strike him down, thus, jeopardizing his plans.
UPDATE 2
Even though the Tablet was not mentioned in the unsealing ritual, it can be inferred from the chapter's panels and the characters' statements and actions that the Tablet was needed for the unsealing ritual, Kishimoto is just being subtle as to avoid a dead giveaway to the origins of Rikudou Sennin being an Uzumaki (Since anyone could just easily deduce the fact that if an Uzumaki Technique is unsealed with reference to an object (Tablet) that was created by Rikudou Sennin, then the latter must have been an Uzumaki himself)
the object in reference was of immediate use to Sasuke's intentions (talking with the Hokages) and immediately after referring to the tablet, the image of the object itself was shown in the next panel. And lighting the torches at the tablet afterwards.
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The mere fact that Orochimaru and others gathered AROUND the tablet is indicative of the ritualistic importance of the Tablet to the unsealing ritual.
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The fact that Orochimaru was facing and crouching in front of the Tablet is indicative of the Tablet's necessity
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The fact that Orochimaru donned the mask in front if the Tablet and crouched and looked at the Tablet at the moment of the RDS Unseal activated is indicative of the Tablet's vital role in the process.
It us only logical to conclude that the importance of the object (Uchiha Tablet) that Sasuke was referring to was of IMMEDIATE USE/IMPORTANCE to his intentions (Unsealing RDS and talking with the former Hokages)
To reject that is to simply deny the elephant in the room, the Tablet was needed for the RDS Unseal.
And if we clearly analyze the situation: What would Sasuke gain from reading the tablet? Sasuke's inquiry is only about the history of the Leaf, anything written on the tablet is prior to that point (Rikudou Sennin vs the Juubi, The Elder and Younger Son, the sealing of the Gedo Mazo in the Moon, the Rinnegan, etc.) everything that is written in the Tablet prior to the Hokages' story is moot to Sasuke's inquiry.
