Until I came to be called [Black Steel King] for slaying a bunch of metal slimes

Synopsis

One day, a large hole suddenly opened in the garden of the house. There was a mysterious slime with a metallic body trembling. Yuuma Mitaka, a dull high school student, spends his days exterminating the "metal slime" that springs up in the hole every morning. As he repeats this daily routine, Yuuma's body undergoes an astonishing evolution! "But this ability... is a bad thing if anyone finds out about it..." His ability is a secret, because it looks like a "monster" when it activates. It's only a matter of time before Yuuma's existence, which is too peculiar, shakes the world...!? An unprecedented Musou fantasy that begins with everyday life! (Source: Fantasia Bunko, translated)