Race: Mysterious Being
Age: Unknown
Gender: Male
Location: Under the earth's crust
Abilities: Superhuman Strength
Weapons: Lava Swords
Affiliation: Mother Earth
Debut: Chapter 4/Episode 1

The Subterranean King is the king of the Subterranean People. He led his people from below the surface to conquer the area on it.

(Source: One-Punch-Man Wikia)