<p>Right, I was doing some research for another discussion and came across something in chapter 663.
</p><p>Tobirama talks about sending Sasuke his soul with his Kinjutsu and apparently like others on this very wiki, made me think of Dan's Spirit Transformation technique. So I have the following theory.
</p><p>What if, like Shadow Clone and Edo Tensei, Spirit Transformation was a technique invented by Tobirama, but, the reason his is a Kinjutsu and not just a ludicrously powerful technique like it is for Dan, is because it was part of Tobirama's arsenal of Edo Tensei techniques?
</p><p>We already know that he can perform Mutually Multiplying Explosive Tags, a technique that he said he used for the first time with his own body, implying that he's done this before with Edos he's created. So what then if Spirit Transformation is just another of those, but Tobirama never bothered his arse to try and use it as a living person, instead using the souls of the dead to assassinate people in case the edo body couldn't do the trick, or whoever he summoned wasn't that strong a fighter or something. And being dead for a very long time, he wasn't around when Dan pulled it off, moving said technique from kinjutsu to regular jutsu?
</p><p>All of this is of course predicated on what is basically fact that, as well as creating this insanely monstrous technique that requires human sacrifice to perform, Tobirama also used in combat enough that he developed at least one, if not two insanely murderous techniques for it and became so infamous with it that a bunch of his dead peers initially thought he had revived them.
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(Edited by Hawkeye2701)