Kinzoku Slime wo Taoshimakutta Ore ga "Kurogane no Ou" to Yobareru made Slime Slayer: From Zero to Black Steel King

Synopsis

One day, a large hole opened up in the yard of a house. There was a mysterious slime with a metallic body that was trembling and shaking. For some reason, Yuuma Mitaka, a dull high school student, spends his days exterminating the "metallic slime" that springs up in the hole every morning. As he repeats this daily routine, Yuma's body undergoes an astonishing evolution! (Source: Kadokawa, translated)