The Anti-Monitor is believed to be The Monitor's counterpart from The Antimatter Universe. He is responsible for the original Crisis on Infinite Earths, he was the sole Guardian of The Sinestro Corps and has now, against his will, become the source of The Black Lantern Corps Central Power Battery.
At the same moment that the universe was born, the antimatter universe was also created. Billions of years later, the being known as the Anti-Monitor began taking power.
He conquered the weaponers of Qward, and from them he learned there was a different universe, a positive matter universe, and he wanted control of it, too.
Little is known about the Anti-Monitor's life before The Crisis began. He is believed to be the The Monitor of the Antimatter Universe, the Anti-Monitor was responsible for the original Crisis on Infinite Earths.
After a cosmic accident tore a hole between the positive and antimatter universe, antimatter began flooding the universe, resulting in the interdimensional "crisis."
It took Earth's greatest heroes to stop the Anti-Monitor, but in the end it was the sacrifices made by the Superman of Earth-Two, Superboy-Prime and Alexander Luthor of Earth-Three that resulted in his death.
When the 52 universes that form the Multiverse were reborn following the Second Crisis, the Anti-Monitor was as well, and he became the Sinestro Corps' sole Guardian and recruited Superboy-Prime, Cyborg-Superman and Parallax to become his Heralds alongside Sinestro.
He promised Hank Henshaw that if he served him, he would end his life. Henshaw, who was grateful for this, built a new War World which the Sinestro Corps could use to transport the Anti-Monitor and their Central Power Battery to Earth.
When he descended upon Coast City, Sodam Yat foolishly rushed into battle against The Anti-Monitor and was swatted away as if he were nothing more than a bothersome insect.
The Anti-Monitor was later seen watching Superboy-Prime engage Sodam (who had just become Ion a few moments earlier) in a vicious fight that would leave Sodam on the verge of death.
During the final battle, the Guardians of the Universe fought the Anti-Monitor at the heart of Coast City. Green Lanterns Guy Gardner and John Stewart made a plan to take down the Anti-Monitor.
They caught him off-guard and managed to toss War World onto his head, causing a giant, but contained, explosion powerful enough to wreck his armor, kill Henshaw and shave The Sinestro Corps' numbers down by a couple hundred members.
The Anti-Monitor survived the blast but was then hurled into space by Superboy-Prime, who wanted revenge for the Anti-Monitor destroying Earth-Prime.
The Anti-Monitor ended up the dead world of Ryut at the edge of Sector 666 where he was taken hostage by Nekron and imprisoned within The Black Lantern Central Power Battery, which is where he has remained ever since.
Recently, Dawn Granger used the powers she had recently gained from her connection to The Entity that embodies The White Fire of Creation on The Black Lantern Central Power Battery to see if her powers would have the same destructive effects on it as it does on the Black Lanterns empowered by it.
A Slave No More
But in doing so Dawn awakened The Anti-Monitor, who had apparently grown back to full size during his time trapped inside the Central Power Battery, and was hit by a powerful surge of anti-matter feedback energy born from his awakening.
While Cyborg was tending to a visibly shocked Dawn, The Anti-Monitor demanded that Nekron set him free and began to claw his way out of the Central Power Battery by warping the dark matter composition of the Battery into a malleable form that would allow him to construct a new suit of armor for himself.
After a quick confrontation with Black Lantern Ice, Guy Gardner found that The Black Lantern Central Power Battery was rupturing open, allowing The Anti-Monitor to begin squeezing his way out.
Knowing that The Anti-Monitor would annihilate everything once he was truly free of Nekron's control, Gardner called for back-up and ordered The Anti-Monitor's extermination, seeing as actually killing him would rob Nekron and The Black Lantern Corps of their ultimate power source.
However, because The Anti-Monitor had yet to become a full Black Lantern, the standard method for killing Black Lanterns was ineffective.
Munk, of The Indigo Tribe, was able to come up with a plan after seeing the effect Dawn Granger's powers had upon Black Lanterns.
His idea was that Gardner would create a distraction by having several Red Lanterns spew their liquid fire upon his eyes while Bedovian of The Sinestro Corps would form a bullet around Dawn and place her into a giant sniping rifle construct.
And upon firing said bullet, all of the Lanterns present would douse the bullet with their light and by doing so would effectively take out The Anti-Monitor for good.
The plan worked and The Anti-Monitor's head was blown apart, his body went into convulsions and eventually slumped down into a lifeless position.
For the first time since the original Crisis, The Anti-Monitor had not only been defeated but killed in the process as well.
But the victory was a bittersweet one, the Lanterns had failed to pull The Anti-Monitor out of The Central Power Battery all of the way before killing him.
Because of this, The Black Lantern Central Power Battery was still capable of using the corpse as an efficient power source for Nekron and The Black Lantern Corps.
Upon realizing the nature of the situation, Gardner and the other Lanterns wrapped a number of tethers around the gargantuan corpse, hoping that they could pull it out of the Battery before it swallowed The Anti-Monitor back up again.
Unfortunately, the Central Power Battery stopped the Lanterns before they could pull The Anti-Monitor's corpse all of the way out by cutting their tethers of light with a dark matter construct shaped like a scythe.
Defeated, the weary Lanterns looked on as The Central Power Battery pulled The Anti-Monitor's corpse back into the cage Nekron had built for him.
A Slave No More: Redux
Later, during the final fight against Nekron, The Entity brought The Anti-Monitor back to life and freed him from The Black Lantern Central Power Battery.
Upon being freed, he lashed out at Nekron and attempted to kill him only to end up being shunted back to The Antimatter Universe.
Recently, Deadman's White Lantern Ring teleported its wielder to Qward, uncloaked him and demanded that Deadman "FIGHT".
The meaning behind these words quickly became clear as Deadman found himself staring up at the ominous form of The Anti-Monitor, who appeared more than ready to crush Deadman like an insect.
Upon seeing him, The Anti-Monitor demanded to know who Deadman was, but before he could answer constructs of those who became White Lanterns during Blackest Night erupted forth from The White Lantern Power Ring and sent The Anti-Monitor crashing into a mountainside.
While The Anti-Monitor was bouncing back from the hit, Deadman asked the ring if the twelve superheroes and villains who had been resurrected after Blackest Night had been brought back to stop The Anti-Monitor, to which the ring replied "NO."
When Deadman realized The Anti-Monitor was getting up, he attempted to use The White Lantern Ring's power to bring him down only to find that the ring would not fire.
The Anti-Monitor then blasted Deadman off the side of a cliff, making Deadman's situation worse as the ring had made it so he could no longer fly.
Taking out the Trash
When Deadman realized that the ring had sent him to fight The Anti-Monitor as a lesson in learning to stay alive, he promised that the ring that he would try.
Knowing his promise rang true, the ring sent Deadman back to The DC Universe, leaving The Anti-Monitor to contemplate exactly what had just happened.
The Anti-Monitor is one of the most formidable beings in the DC universe, having caused more deaths than any other villain the DC universe has encountered. He has destroyed and absorbed thousands of positive matter universes. The Anti-Monitor has immense strength and durability.
He was able to shrug off blows from Superman and stalemate the Spectre. He was able to defeat hordes of superheroes. He is able to alter reality, fire bolts of energy, time travel, and is more powerful than his counterpart, the Monitor. He can also absorb his own shadow demons to recharge himself. It took all of the Multiverse to destroy him.