While hashirama was hokage, Tobirama spend most of his time on invention different justus.

Experimenting, practicing, gathering information, reading every books about jutsu and chakra that he had access too even the forbidden once.

He had to keep himself busy to prevent the torturing thoughts from coming to his mind. Thought of previous war between senju and Uchiha clans. People they lost....and of course Izuna's death that was heavy on his shoulders.

Losing dear people; that hurts a lot of course. And that caused an idea "bringing the dead one back to life" it was impossible of course but with chakra and jutsu nothing looked impossible for him. Or maybe he desperately wanted to bring someone back; someone he lost.

After a lot of experiments, practice and gathering information he finally invented Edo Tense. And who he brought back wasn't his brothers or anyone else it was an Uchiha; Izuna uchiha

Now he was looking at the Uchiha boy in front of him with wide eyes who was on his knees slowly opening his eyes.