Promethium began as a human ruler on La Metal. As the artificial sun faded, she commanded the conversion of life forms into machines for survival, defying opposition from her husband Dr. Ban and daughters Maetel and Emeraldas. An experiment gone awry, caused by interference, critically failed. This transformed Promethium into a cold, machine-based entity who forcibly assimilated others, including scientist Hardgear, to consolidate power. Her mechanization drive became a campaign of conquest masked as voluntary choice.

After exiling Maetel and Emeraldas, Promethium ruled as the tyrannical Queen of the Machine Empire. She later forged an artificial planet, Planet Promethium, to perpetuate her dominion after her original body's destruction in a rebellion. Her tactics encompassed deploying a planet-destroying superweapon and manipulating Maetel with an offer of queenship—intended to make Maetel a puppet ruler. Promethium's concealed goal remained universal control through mechanization, sparking civil war when exposed.

She confronted Maetel and allies like Captain Harlock directly. Her end came when Maetel deceived her using Dr. Ban's consciousness capsule, triggering an energy backlash. This annihilated Planet Promethium and all systems capable of her restoration, ensuring permanent demise. Promethium's legacy endured through the Machine Empire's ideology and the energy-draining systems on planets like Great Andromeda, harvesting humans to sustain machine life.