Chapter 128:
Senjutsu is a state where one can utilize Sage Chakra. Its power far surpasses that of the normal state, sometimes by several times or even dozens of times, while also granting heightened sensory abilities.
The reason Orochimaru suddenly brought up Senjutsu was that, when he was contemplating power, a crucial question crossed his mind:
Can I learn the Senjutsu of the First Hokage?
It was a reasonable line of thinking. The First Hokage, Senju Hashirama, likely utilized the Sage Art from Shikkotsu Forest!
If that were true, then wouldn't it be impossible for him to learn the ultimate abilities of the First Hokage?
"Yes!"
As expected, Orochimaru's suspicions were confirmed when the Slug from Shikkotsu Forest answered directly.
This was precisely what Orochimaru had anticipated. In his memory, there were three great holy places for Senjutsu: Mount Myōboku, Ryūchi Cave, and Shikkotsu Forest. While the existence of Senjutsu from the latter had never been explicitly mentioned, it was well-known that Senju Hashirama was capable of it. However, it was never made clear where his Sage powers originated from.
Given Hashirama's connection with Tsunade, it made sense that his Sage Art came from Shikkotsu Forest.