Tobirama had been listening to his brother, the first hokage and his brother, hashirama Senju, talk about how he had recently found out about another clan that existed away from the 5 nations of the ninja world, it is said that they are located outside of the fire nation where the village they created was built at, the hidden leaf village.

Tobirama listens to his brother talk about the clan, talk about how that clan is said to be one of the strongest clans though they do not have chakra