The Devil's Cage - Chapter 73: Competition

When The Cute Kunoichis Spied on Their Teacher's Secret is a light novel written by Kazuma Kamachi and illustrated by Ryuushi Tajima.

The story takes place in a world where Japan never ended their isolationist phase, and agents from foreign powers are always threatening to steal the country's advanced technology. In order to combat this threat there has been a resurgence of ninjas that use Strategic Martial Arts, techniques that combine cutting-edge modern technology with a traditional foundation of martial arts and ninjutsu. The story follows Souha Sugiyado, a 17-year old ninja instructor and former ninja prodigy who was forced to retire from active duty by the wounds he suffered in his last mission. Souha's four students are all kunoichi who have all gained the rank of Elite Ninja, the highest official rank available to ninja. However, one day two Shogunate ninja approach him with the news that his four students have gone rogue and have stolen a portable modular nuclear reactor, and thus they ask for his help in bringing his students to justice.

[Please transform.] [The penalty for disobedience is death.] I am saying this because I have been wronged. Help me!

My surname is Wu, and everyone calls me Xiao Wu.

Listen, brother, here are some of my experiences-

Don’t listen to what it says when you fight with the dragon, just fight until it dies;

It’s a little more troublesome to beat the goblin, it’s better not to let it see you, you have to shoot the black gun;

It’s a word to engage in orcs, naked!

If the devil dares to come to trouble, he will break his horns, and he will kneel;

Okay, I’m going to fight at the bar now, I’ll write more about the introduction when I get back

- Description from novelbuddy

He is a young man without a spirit root. It is believed that this denies him the chance at cultivation. However, a mysterious lady imparts a Supreme Demon Classic to him and from then on, he starts his path of cultivation.
He undergoes a total transformation to emerge as the most fearsome and powerful fiendish demon in his era, that even immortals and fiends are fearful of him, and saints are at his beck and call.
Mastering the Supreme Demon Classic is the turning point in his life. He is able to unleash his immense and divine power from within, illuminating the entire universe!