Subterranean King

The Underground King (地底王, Chitei no Ō) is the king of the Underground People. He led his people from below the surface to conquer the area on it. He was however quickly defeated by Saitama.

Appearance
Like the Underground People, the Underground King has a humanoid appearance, but is about double the size of a regular human being. He is covered with black, armor-like pieces, with lava running in the space between them. He has three, sharp eyes and four arms, all with only three fingers.

Personality
The Underground King seems rather confident, but is defeated almost instantly.

Introduction Arc
Saitama has a dream where he is attacked by the Underground People, and after a hard battle where he defeats them the Underground King shows up. Right before they start to fight, Saitama's dream ends. When the Underground King does show up, he is defeated instantly.

Abilities
Nothing is known of The Underground King's powers due to his quick death but it is presumed that he is a skilled swordsman.

Equipment
Swords: The Underground King carries four very large flaming swords, one for each arm.

Trivia

 * It is worth noting that he is the first of three "Kings" mentioned in the series, the other two being the Sea King and the Sky King.