In the beginning
Billions of years ago on the planet Oa in the center of the universe, a scientist named Krona dedicated his life to probing the mystery of the origin of the universe. Despite the warnings and protests of all his fellow scientists - who cited legendary warnings foretelling the end of the universe should its secrets be discovered - Krona forged ahead with his work until, after thousands of years, the immortal was able to pierce through the time barrier to see creation's very beginning.
At that instant, a cosmic bolt of energy destroyed Krona's time spanning device. Krona survived, but his probing had disastrous results. The feedback from the destruction of the device created a myriad of parallel universes, all but one of them mirroring the universe of Krona. These infinite universes, known as the Multiverse, were, except for variations stemming from the growth of civilization in each one, identical to the original.
Had this been the only result of Krona's experiment, the multiverse could have rested easy. However, at the same instant another universe came into being - the Anti-matter Universe. Unlike the other newly formed planes, this one was the antithesis of all known creation. Not only was it a universe composed of Anti-matter, but it was a place of evil also, and it had the distinction of being the only plane that had replicated the planet Oa in a world known as Qward.
As punishment for his deeds, his fellow Oans turned Krona into disembodied energy and sent to harmlessly circle the universe forever. In penance for what they felt was their neglect in failing to control Krona, the Oans debated among themselves and resolved to watch over and protect the universe, becoming the Guardians of the Universe, eventually creating the Green Lantern Corps. A splinter group went off on their own and evolved into beings called the Controllers.
Meanwhile, on the otherwise lifeless moons of Oa and Qward, another form of creation was taking place. On Oa's Moon, a being was born who became known as the Monitor. He rose from the heart of the moon, unknown and unnoticed by all. He spent the early part of his existence in meditation, utilizing the great power within him to reach out to the universe around him and learn all his secrets.
Simultaneously, as if in answer to some cosmic need for balance, a being also arose from the moon of Qward in the Anti-Matter universe - this became the Anti-Monitor. The Anti-Monitor was not content with his own company and the search for knowledge. Instead, weaned on the power of the Anti-Matter universe, he expanded his reach and grew to great power, taking control of Qward and creating the dreaded Thunderers of Qward. Over the ages that followed, the Anti-Monitor's power grew and spread across the whole of the Anti-Matter universe. He might have been content with complete domination and mastery of his own universe, except that the Anti-Monitor began to sense the existence of a being of comparable power to his own on another plane.
With that awareness came the need to act, to counter what the Anti-Monitor believed was a threat to his existence and his power. Thus began a war between the two mighty beings across the vibrational barrier that separated their universes. Since both were of equal power, neither could win this cosmic war, and the battle was fought for one million years until a simultaneous barrage from both participants rendered both unconscious and immobile, a state they remained in for nearly 9 billion years.
Billions of years after the end of that battle, on an Earth in one of the planes in the Multiverse, a brilliant young scientist began his own search into the origins of the universe. The scientist ventured into the Anti-Matter universe and his interference caused not only the destruction of his own universe, but also led to the freeing and awakening of the till now unconscious Anti-Monitor. He also opened the way for the Anti-Matter universe to expand to fill the void left by his destroyed universe. The Monitor was also awakened by this cataclysmic event and sensed the presence of his evil double who was now free to reach into the positive matter universe and work his destructive power. The Monitor also sensed the existence of the young scientist responsible for the Anti-Monitor's freedom, who had survived the death of his universe. The young scientist became sort of a pointer toward the location and activities of the Anti-Monitor. His powers were spreading rapidly across the multiverse and his Anti-Matter universe was devouring uncounted millions of planes of existence as it expanded. The young scientist took the name Pariah because of his fate to watch worlds die while he watched helplessly.
The Monitor knew that he would not survive long enough to see his evil counterpart destroyed, but could set in motion a complex series of events that would, perhaps, lead to the Anti-Monitor's defeat before his Anti-Matter universe devoured the entire span of the multiverse. In pursuit of this end, the Monitor began to observe and catalogue the powers of all the super beings on all known surviving worlds, learning all he could about their vast and varied abilities, seeking those who would help save the multiverse. The Monitor knew his time was limited. He had rescued and raised a young woman named Lyla from a shipwreck, and she acted first as his assistant, and later came into full power as a being known as Harbinger, imbued with power by her mentor and forced, eventually, by circumstances beyond her power and understanding to be the direct cause of the Monitor's death. His death by her hand was, however, all part of the Monitor's plan, as it allowed his life force to be freed and sent to power his mammoth "vibrational forks," devices that were created to stop the collapse and eventual destruction of the multiverse.
With the Monitor dead, the Anti-Monitor believed he had the way clear to complete the destruction of the positive matter multiverse. The Monitor had set into motion his scheme, which drew the heroes of the five surviving worlds together to defeat the Anti-Monitor's plans. Out of the Monitor's being was created a netherverse that absorbed the dimensions of Earth-One, Earth-Two, Earth-X, Earth-Four, and Earth-S, temporarily protecting them, although they were headed on a slow but inevitable collision course that would destroy them all.
With his evil army of Thunderers and Shadow Warriors from Qward, the Anti-Monitor entered into battle with the heroes, meeting with defeat after defeat at their hands, despite his god-like powers. Yet, the situation remained a virtual stand off until the heroes ventured back to the dawn of time in pursuit of the Anti-Monitor. There, in a battle with the Spectre, the Anti-Monitor was apparently defeated and, in the next instant of awareness for all the participants at the battle of the beginning of time, the world was back to normal. Or nearly so, because where there had existed many worlds with many variations, there was now only one world, combining elements of all the surviving worlds. To all intents and purposes it appeared that the multiverse never existed, and none but those who were in the battle could recall the past.
However, the Anti-Monitor survived. He made a final appearance, facing the surviving heroes and Alexander Luthor Jr., the son of Alex and Lois Luthor, the only being in the multiverse to be part positive and part anti-matter. In a final battle with Luthor, the Superman of Earth-Two, and the Superboy of Earth-Prime, the Anti-Monitor lost his life.
In the final issues of the DC Metal event, it is revealed that the Monitor is still alive, but was weakened from previous crises, allowing The Batman Who Laughs to capture him and use his positive energy to destroy the DC Multiverse. After the Batman Who Laughs' plan is foiled, the Monitor is set free and puts Barbatos in chains.
Infinite Crisis revealed that there are many Monitors. Each Monitor is assigned a universe to protect. As of DC Rebirth, it can be assumed that they are no longer active, due to the revelation that they were splintered aspects of Mar Novu.
In Other Media
The Monitor makes his live-action debut in the crossover event "Elseworlds". He allows John Dee to alter reality in order to prepare the Earth's heroes of an upcoming threat.
In the seventh season finale, Mar Novu appears to Oliver Queen, telling him that he will assist him in averting the crisis, and that ultimately, Oliver will have to sacrifice himself to ensure the lives of millions.
The Monitor appears in the season four finale, retrieving Lex Luthor's body and releasing Malefic from the Phantom Zone.