MTL - Sinister Ex-Girlfriend - Chapter 20 : The Ninth Nightmare (4)

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

Wendy Waltz, a diehard anti romantic! Though born as royalty, she had been neglected by her father and bullied by her stepmother and younger sibling. To make this worse, she had been betrayed by her lover. She throws away her royalty and name and starts anew with a new name and a strange power! But who is this Head of the Imperial Guards, Lard Shroder, who keeps appearing beside her?

Demons are rampant, the court is corrupt, the human race is declining, demons are rampant, and the world is treacherous and haunted.

Martial arts masters are just more delicious blood food in front of monsters.

There has never been a savior in this world!

My name is Jiang Heng, and I am the one who runs amok!

- Description from novelbuddy

He became a teacher at the best magic academy in the empire.
….For being mistaken for someone else.