As for why they did not turn into stone toads or something similar!
It was simple. The methods of training in Sage Mode that currently existed in the Shinobi World had all originated from the Three Great Sage Lands.
Those forms of Sage Mode were suited to those creatures' own cultivation, not to a normal human cultivation system.
But humans stood above all other living beings. There was no reason those creatures could cultivate into sages while humans had no path of their own.
Yechen strongly suspected that a great catastrophe had once struck the Shinobi World, leaving those people dead or scattered, until their kind no longer appeared.
The so-called Three Great Sage Lands were merely groups of nonhuman beings that had managed to cling to survival.
Perhaps beings such as the Shinigami and the Evil God represented humanity's own path to becoming sages. They had simply lost control, just like those who turned into stone toads while practicing Mount Myōboku's sage arts.
If that was the case, the Shinigami might be the result of losing control while practicing human Senjutsu-a soul entity.
With that conjecture in mind, Yechen began considering practicing this technique. It was not to use against enemies, since casting it cost the user their own soul.
He only wanted to practice it to see whether he could summon the Shinigami.
If the Shinigami was indeed a soul entity, Energy Conversion could suppress him. Perhaps this would give Yechen another way to obtain evolution points.
For now, however, he had no intention of trying. The Shinigami was far too unusual, something entirely beyond his understanding. Until he had some confidence, it was better not to act rashly.
He turned his attention to the third forbidden technique. It was precisely the Impure World Reincarnation he wanted, but