Dorei Senshi No Isekai Taneuma Seikatsu summary:

Yes, everything is exactly as you expected when you saw the cover."Ryuu", a former modern human reincarnated in another world. He was a slave in this world. However, once the knowledge of his previous life was revived, his magic power was 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. Just then, someone who bought Ryuu appeared. She was the noble Juller, the Juller Family, who saw slaves as power, wanted to own a great slave. And the first instruction given to Ryuu as a slave was to "seed" a beautiful female slave...!I'm a Slaves but super free!　This otherworldly fantasy, which starts from the lowest class but has an unlimited future, start with a BANG!