Shoujo Grand Summoning - Chapter 12: Flat-chested, acrophobic, Kaichou sama!

One of the seven strongest people on the continent, the King of Mercenaries. He started a war to avenge his fallen family and destroyed territory but failed and lost his life. However… “Wow, I’m alive?” I returned to the past, back through time. A perfect opportunity to right my regrets and reverse everything. It doesn’t matter if people around me point fingers, calling me a scoundrel, or dismiss me as tr*sh. Because… “I’ve got a plan.” “What plan?” “A plan to destroy everything.” There won’t be a second failure. This time, I’ll wipe out all my enemies. …But first, I need to rebuild this damn estate.

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.

Ethan finds himself mysteriously transported to another world, where the strong reign and the weak perish.Welcomed by a kind farmer who decides to help him upon seeing his poor condition, Ethan begins to hope for a new life.However, his dream is shattered when an acquaintance deceives him, leading to his sale at the slave market. Desperate and confused, Ethan wonders what he has done to deserve all this.“Which deity have I offended to deserve all this?”Just when he thinks all is lost, fate has another surprise in store for him, once again changing the course of his destiny.

He rose from abjection and achieved eternity in glory.
He embraced the prettiest beauties and killed the strongest opponents.
He was able to shift the clouds into rain and cover the sky with one hand.
He could overrun the entire world and go anywhere as he pleased.
He pacified wars and held the world in awe.
He was matchless in the world.
However, his hair turned white overnight for his beloved woman and he was on the verge of death.
He was surrounded by enemies who were trying to take his life.
He struggled his way to become mighty and fought for everything he cherished.
For this age, he was the only king of the Dark World.
His name was a byword for might.
As a man living in the world, his only ideal was to pursue brilliance.

- Description from Novelupdates