Bear with me for a second.
Look at this manga scan:
Hashirama has knowledge on Rinne Tensei. Arguably the most secretive of techniques the Rinnegan offers. If he knows about that, who's to say that he doesn't know more? The mere fact that he knows about it is astounding probably leading to the conclusion that there is also something reminiscent of the Uchiha tablet on the Senju Side; I mean, Rikudou chose his Younger son for a reason...
And arguably the best asset for Madara and his Rinnegan is Preta, and to an extension, Deva Path. Preta is a non-factor since Mokuton is actual trees and not pure chakra. Deva is more dangerous but Shinsenjuu has more destructive power.
Look at this manga scan:
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Hashirama has knowledge on Rinne Tensei. Arguably the most secretive of techniques the Rinnegan offers. If he knows about that, who's to say that he doesn't know more? The mere fact that he knows about it is astounding probably leading to the conclusion that there is also something reminiscent of the Uchiha tablet on the Senju Side; I mean, Rikudou chose his Younger son for a reason...
And arguably the best asset for Madara and his Rinnegan is Preta, and to an extension, Deva Path. Preta is a non-factor since Mokuton is actual trees and not pure chakra. Deva is more dangerous but Shinsenjuu has more destructive power.