Hashi/Minato's Shiki Fujin and Tobirama's Edo Tensei
The purpose of this thread is to theorize how Hashirama or whoever taught him discovered Shiki Fujin and how Tobirama discovered Edo Tensei.
Shiki Fujin summons an actual shinigami which eats the user and the victim's souls, sealing them in it's belly. A shinigami in Japanese culture is like the grim reaper, his job is to make sure a person moves on to the next world when they die, but for some reason if you know how to do this jutsu u can get your own shinigami to eat people instead of allowing them to pass to the next world. The only logical explanation for this that I can think of is at some point a living person came in contact with a shinigami and was told about this contract technique.
We have only seen Konoha elite able to use this technique, be them Senju or supporters of the Will of Fire. This happens to be the perfect technique for the Senju clan, the descendants of the younger son who believe love is the key to peace, because it is always used to sacrifice yourself to save others.
With that said I believe there is a possibility that the technique was left to the Senju clan by the so6p just like he left the tablet to the uchiha. After all, who better to meet a Shinigami in person than a guy who could create life.
Now on to Tobirama and Edo Tensei.
Why would Tobirama seek out Edo Tensei? There must have been someone close to him whom he was consumed by the idea of bringing them back to life.
How did he create it? I don't think he sat around a lab just killing animals, smearing DNA and performing hand motions until something happened. The most logical explanation again is that an entity like a Shinigami told him of this technique.
How he would have met a shinigami I have no idea.
Let's also throw the rinnegan and the King of Hell into the mix. Somehow having the rinnegan can give you Shinigami like powers of being able to send people's souls to the pure world or pull them back from it by summoning the actual King of Hell to do these things. It would seem that a living person can become so powerful that they can affect the pure world whilst still alive in the impure world. This line of thought could lead to the possibilities of shinobi being able to escape the pure world on their own accord.
Does anyone else have any ideas on how these techniques were created?