I've Become a Genius Actor - Chapter 48:

On the desolate barren planet of Rennes, an Asian teenager named Lin Yuan receives an admission notice for the Empire’s best military academy San Romia. So begins his life changing military school career.

Among the demons, the supreme ones are called “Great Saints.”

A young man leaves his mountain village and travels around the world. By unceasing efforts, he finally becomes a legend of the age.

- Description from Novelupdates

Yeon-woo had a twin brother who disappeared five years ago. One day, a pocket watch left by his brother returned to his possession. Inside, he found a hidden diary in which was recorded “By the time you hear this, I guess I will be already dead….”