Orochimaru did not use the Shiki Fuujin Kai (Reaper Death Unseal) before becauseBecause Orochimaru needed theto open theand have someone (his lackeys) put theTake a look at this:

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jutsu

Suigetsu said "So you need ato open that door"Notice how the Marble door has thein it instead of the usual Uchiha Crest Symbol?

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And did you noticeafter opening the marble door?

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it is because:

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It makes perfect sense thatto the secret room, it's because it was also used for, 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 thestage of his Sharingan during the massacre when he broke free from Itachi's genjutsu that is why he was able to enter the Shrine

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did not say

Experiments

And Kabutohe or Orochimaru had access to the Secret Room or the Tablet and ready/study its' contents, they merelyits' contents (they knew its' existence but couldn't access it)(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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Theis this:

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The

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someone else

So this is, it's because heOrochimaru researched on the Reaper Death Unseal

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SoThat is why Orochimaru didn't use the info. on the scroll years and years ago:

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It was because hehe needed the Sharingan to open the door,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 theat anafter losing his arms to RDS, but he still couldn't use it.

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He needed Sasuke'sbut 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, butbecause his body transfer technique was locked for another three years, so he tucked it neatly in his

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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,of the Reaper Death Unseal Jutsu:

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to open

Orochimaru needed the Sharinganthe Naka Shrine sealed door where the Tablet was, he neededin 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 toBut he suffered

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[Orochimaru intended to possess Sasuke's body to, but he probably intendedeither Kabuto or some other Sound Ninja to wear the mask in front of the tablet to perform Reaper Death Unseal and die, since, 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 becausea.) Orochimaru was a non-Uchiha so Sasuke would not allow such a disgraceful character like Oro to defile the sanctity of his clan's shrineb.) Sasuke knows full well of Orochimaru's(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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The Naka Shrine Tablet was the catalyst

the Reaper Death Unseal.

Orochimaru(after performing his body transfer technique, since three years was the time limit before he could possess a new body again) toSo that meansforSomething toabout:We know that after the Tsunade Retrieval Arc, thefollowed.- Many assumed thatOrochimaru 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,- Because even if Orochimaru was successful in obtaining Sasuke's body during that time,. so Orochimaru- So thefor 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 transferSasuke arrived at his lair, thushis technique forwhen he was ready to possess Sasuke's body,, thus, jeopardizing his plans.