Dorei Senshi no Isekai Taneuma Seikatsu

Story:

"Ryuu" is a former modern human reincarnated in another world. He is a slave in this new world. However, once the knowledge of his previous life was revived, his magic power is also awakened. He gained strength at a speed that was incomparable to other slaves, and he easily defeated strong opponents, becoming the strongest gladiator among slaves. Ryuu is bought by the noble Juller. The Juller Family, who saw slaves as power, wanted to own a great slave. The first instruction given to Ryuu as a slave was to "seed" a beautiful female slave…! I'm a Slaves but super free!