The dragon's harem - Chapter 1067: Lala’s Research

"Locke transmigrated into an era where everyone had Battle Beasts. Daemons were rampant and humans were forced to build walls around their dwellings while depending on Beastmasters to keep those monsters at bay.

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

Roy died during a war against the Evil God Cult, but somehow, he returned back in time to 30 years in the past where he was just a boy who awaken his summoning magic.