The OverLord of Darkness is referred as the creator of the human race and is the source of the Lords, monsters that protect humans by force if needed.
Long ago, the Overlord of Darkness coexisted with his "twin" the Overlord of Light as they created the world. However, when neither could agree on who would reign over their creation, OverLord of Darkness triumphed over his brother in the resulting battle. But in his dying breath, the OverLord of Light bestowed his essence upon humanity in the hopes that they would evolve beyond his brother's control. Refusing to allow such advanced humans to exist, the OverLord created the Lords to kill all humans who possess the potential to "evolve" into Agito while dealing with the Grongi Tribe who have been preying on humans for their game.
Though he died around the Pleistocene era, the OverLord of Darkness left a puzzle that held a model of his very own DNA. Once the puzzle was solved, the OverLord's DNA was reconstructed from scratch and he reformed his physical body with his rebirth was witnessed by Sakiko Mikumo. Aging to an eight-year-old, the OverLord followed her until she was killed by Anguis Femineus, whom the OverLord punished for slaughtering a normal human. Encountering Ryo Ashihara as he was near death, the OverLord used his power to prolong his life, assuming his adult form as a result.
Forced to flee after being nearly killed by a fear-driven Gills, the OverLord took it upon himself to personally murder Tomoko Miura. Arrested and placed in a mental ward, the OverLord shut down from the burden of personally killing a human and came to the conclusion to watch from the sidelines as the Lords, called the Unknown by humans, committed various supernatural murders on anyone reaching paranormal maturity, though he intervened once to give Agito's Tornador a power boost. It would later result with Shouichi Tsugami, along with Ryo and Makoto Hikawa, further complicating matters and interfering in his emissaries' work.
After losing the El of the Water, the Overlord realized that Shouichi, Ryo, and Kaoru Kino have all grown too powerful and decided to strip them of their Seeds of Agito. However, the OverLord realized he needs to kill the Riders in order to quell the Seeds he already absorbed but loses them when Shouichi punches him in the face. That event left an impression on the OverLord, convincing him that humanity have got beyond his control and that only the systematic genocide of all is required.
The OverLord would put his genocide of humanity into action: realigning the constellations of the Zodiac, ultimately killing off all of mankind in bizarre doppelgänger homicides starting with those born under Scorpio. As the process requires tremendous concentration, the OverLord creates the El of the Wind to be his bodyguard and the El of the Ground to continue killing those reaching paranormal maturity. But the Riders make one final stand and with the El Lords destroyed, the OverLord attempts to retreat but is wounded by Agito's Shining Rider Kick.
The OverLord survived the attack and has a conversation with Tetsuya Sawaki before the latter died, the latter's words convincing him that while he created mankind, the OverLord didn't truly understand them. After which, he hints that he decides to postpone his plans and observe how humans will use their own gifts and whether or not they are worth of existing.