Subterranean King

The Subterranean King (地底王, Chitēō) was the king of the Subterranean People. He led his people from below the surface to conquer the area on it. He was quickly defeated by Saitama.

Appearance
Like the Subterranean People, the Subterranean King had a humanoid appearance, but was about double the size of a regular human being. He was covered with black, armor-like pieces, with blue lava running in the space between them. He has three, sharp eyes and four arms, all with only three fingers. For each one of his arms, he carried a magma blade of blue color.

In the manga, his armor is brown in coloration with yellow/reddish magma swimming through its body which matches the common known colors of an active volcano. His blades also have the same color of magma.

Personality
The Subterranean King seemed to be rather confident in his abilities, but was defeated almost instantly.

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

Monster Association Arc
During the fight between Child Emperor and the resurrected Phoenix Man, a statue of the Subterranean King is seen, along with two centipedes encircled around him. His people are also revealed to have been killed by the Monster Association.

Abilities and Powers
Nothing is known of the Subterranean King's powers due to his quick death but it is presumed that he is a skilled swordsman. Considering that he has a status similar to the Deep Sea King and the Sky King, it can be assumed that he is extremely powerful, as he was the ruler of an army of powerful subterranean Mysterious Beings (but still, no match for Saitama).

Fighting Style
Swordsman: Given his choice of weapons, the Subterranean King likely has some swordsmanship skills. It is unknown how skilled he actually is, due to his quick death at the hands of Saitama.

Equipment
Swords: The Subterranean King carries four very large flaming swords, one for each arm. His swords emit powerful heat like lava and each cleave cuts and burns his enemy.

Trivia

 * The Subterranean King is the first of many "kings" mentioned in the series, the others being the Beast King, the Deep Sea King, the Ancient King (anime-only), the Sky King, the Monster King, the Forest King, and Pluton, King of the Underworld.