Rise of the Dark Alpha - Chapter 323 - Doubts & Deeds

Raised to be the perfect soldier for battle, Alexander had risen to be the youngest S rank Merc. After attaining the strength he so desired, his adoptive father had forcefully made him leave the battlefield. Alex unwilling to acknowledge his retirement was given a mission by his adoptive father. "The mission objective is to find a girl in the public High School you'll be attending and protect her."Triggered by the words mission objective, Alexander who had a 100% completion rate, accepted the conditions of the mission. Follow Alexander in his weird, almost comedic high school life.Alex's father: Hey Alex, did you get to school properly?Alex: Affirmative, I reached the designated place on time.Alex's father:So why did you call?Alex: The target I have picked is situated in classroom 1-B, while I'm in classroom 1-A. Asking permission to threaten the Principal into placing me in classroom 1-BAlex's father: DENIED!...

Calamity descends upon a a tranquil world in the black of night, transforming it into a death game! This a a true apocalypse, but this is not the end of humanity!

- Description from Novelupdates

Known as the country’s top onmyoji, Yuuma Kisaragi, who lived to the age of 99, awoke to find himself in an unfamiliar forest. A voice echoed in his mind, revealing that he had been reincarnated into another world at the age of 20.“Good morning, Master. Skip the greetings and please defeat the goblin in front of you.”The voice belonged to a skill acquired during reincarnation called “AI,” designed to assist Yuuma in his new life. When the skill, AI, took over the form of a golden-haired fox yokai, the familiar shape of Yuuma’s shikigami, he decided to embark on a new life alongside AI-chan.The onmyoji and AI-chan’s second life began, tackling various challenges and formidable enemies with the strongest spells and shikigami.

Our protagonist Caam is a man from Earth that was reincarnated into this world as a demon. In order to survive, he spared no effort in avoiding the sparks of battle and without noticing it, he became strong himself.