Starting with the Uchiha clan, they began cultivating demonic arts.

As an unofficial Uchiha with only 1/4 of the bloodline, Uchiha Touyu said that he didn't really want to be a demon, but the Uchiha clan insisted on making him a ninja.
It's one thing to become an older ninja at the ninja academy, but to graduate right before the Ninja World War? What's going on here?
There's no other way. Since they're forcing me to die anyway, I guess I'll just have to become a real demon. I'll use spirit-binding techniques to lure souls, refine my bloodline in the Myriad Spirits Blood Pool, and kill to forge demonic weapons... This damned world is lacking in everything except ninjas who are ready to die!