Subterranean King

Standing at roughly twice the height of an ordinary person, the king carries a broadly humanoid shape plated in interlocking black armor, with a glow of blue lava seeping through the gaps between the plates. Three slim eyes sit in his face, and four arms branch from his body, each ending in a three-fingered hand that grips a blade of red-hot magma. The manga renders him differently, wrapping his frame in brown plating with yellow and reddish molten rock churning inside, a palette drawn straight from a living volcano, and coloring his swords to match.

What little the king reveals of himself is pure bravado. He struts onto the battlefield brimming with certainty in his own power, only for that confidence to evaporate almost the moment the fight begins.

Ruling over the Subterranean People from beneath the earth's crust, the king judged that his overgrown nation should claim the world above and marched them toward the surface. In Saitama's dream, a grueling war against his subjects ends just as the king steps forward for a final duel, and Saitama wakes before the two can trade blows. Discovering that the invaders are real, Saitama eagerly heads out to meet them, and when the actual king appears he is felled in a single instant, sending his people scrambling back underground in surrender.

Very little is confirmed about his combat ability given how quickly he died, though wielding four flaming swords at once implies some grounding in swordsmanship. Each blade pours out a lava-like substance so that every slash both cuts and scorches. Officially rated at Demon level, he is placed on par with the Deep Sea King and the Sky King, and he commanded an army of formidable subterranean Mysterious Beings.