Created by the Guardians as their first attempt to enforce their will and guard against all menaces, about 3.5 billion years ago, the Manhunters went rogue when they were re-purposed as weapons of fear by one of the Guardians, who was possessed by a cosmic entity known as Parallax. In order to battle the Manhunters, the Guardians took extreme measures, as the rings powered by the emerald energy of will were not enough so they created the yellow rings, that are powered by fear. Every time the Manhunters seized a planet and started to exterminate the population, the new Yellow Lantern Corps arrived and powered by the fear of the population got much stronger.
In another place in the galaxy, something detects the multiple Green Lanterns on sector 2814 and thus the terms of the agreement have been violated. A mysterious voice says that it was only a matter of time and then sends the Manhunters on Earth as a way to quell the resurgence and let them know the meaning of fear.
At S.T.A.R. Labs, an employee of the lab tells Emil Hamilton that a signal was detected from outside of the facility, similar to that of a power ring. John Stewart says he hasn't invited anyone from the Green Lantern Corps there, so he and newest Corps member Clark Kent change back to their Lantern costumes and go to check what is going on. Much to their surprise they find an army of Manhunters waiting outside.
Superman and John Stewart try to contain the Manhunters' attack without putting in danger any civilian in Metropolis. Since they can not beat them, as their rings can't make a dent in them, Clark quickly decides to take the fight to a non populated area to avoid causing any casualties, so they fly to the Moon, where Justice League's new Watchtower base is still being built by S.T.A.R. Labs' drones. While fighting with them the two heroes hear the Manhunters saying that the Green Lanterns have violated a treaty, but Stewart says he has no idea what they're talking about. To aid Superman and Green Lantern in their battle, Tess takes under her control the drones and commands them to attack the Manhunters. During the battle, John realizes that the Manhunters are powered by yellow rings and that is why their rings can not work on them so he tells to Clark to fall back. Superman tells to Tess to stand down and then ends the battle with a powerful blast of heat vision destroying the remaining Manhunters. Clark asks John if this is the last of them and he says there's always more.
On a parallel Earth, Superman and Lois Lane are fighting some Manhunters, who are chasing them so they can kill them. After destroying some of them, Superman takes Lois and fly away escaping. Lois puzzles over why the Manhunters were hunting them given the fact that the Green Lanterns are their usual opponents but Clark says these robots were running on Bleed instead of Lantern energy, suggesting that the Monitors probably use them to deal with the "supers" and "metas" of each Earth while they move their ships into firing position.
Superman is fighting with the Manhunters, preventing them from attacking the refugees. The Manhunters suddenly evacuate in preparation for the "purge."
Across the Atlantic Ocean, Clark and Lois are almost home, watching the Monitors rebooting the universe which Lex convinced them to do as they break the water and air down to base elements and collecting it as they prepare to reassemble it, realizing Lex may have bought them some time to stop this. Clark wonders if Lois is ready to see this but Lois rationalize since they are possibly one of the last journalists, they have no choice. Lois wonders if her dad and his mom, along with everyone else in D.C. being vaporized is better than being terminated, Superman says he doesn't plan on finding out and would only take time to grieve if they can't save the day. However he tells tells her they need to focus because one of the ships releases a group of Manhunters on them.
Superman delivers a rousing speech to the heroes. The other Earths might have been erased or rebooted but they are determined for their Earth to survive and their continuity to continue. With that Superman, Luthor and all of Earth's heroes attack the incoming Manhunters.
Superman spots a pod approaching Earth, feelings of sheer horror aside, he, Kara and J'onn try to stop the pod from hitting its target. Even with their combined might the pod overcomes them and speeds beyond them breaking the sound barrier.
Lex is confronted by the Monitors and Manhunters, for the first time ever with no backup plan and no way out. Before Lex is killed a truck slams into his attackers crushing them, and out climbs Otis from the wreckage. Lex is for once pleased and grateful to have Otis as a friend only then to see the hapless friend incinerated by a laser of a barely functioning Manhunter.
All the Manhunters later are destroyed after the Monitors' defeat.
In the Comics
The Manhunters were the first attempt of the Guardians of the Universe to create an interstellar police force that would combat evil all over the cosmos. Their name and much of their code of behavior was modeled by the Guardians of the Universe on the Manhunters of Ma'aleca'andra (Mars) millions of years ago. For thousands of years, they served the Guardians well. However, the Manhunters became obsessed with the act of "hunting" criminals. Their code, "No man escapes the Manhunters" (corrupted from the original), became more important to them than seeing justice done.
Eventually, the robots conspired to rebel against their masters, but the Guardians defeated and destroyed most of them. Those that survived hid away on many planets, slowly rebuilding their forces and spreading their beliefs to others. Since then, the over-riding goal of the Manhunters has been to take revenge on the Guardians, as well as on their replacements, the Green Lantern Corps.
Years later, it was revealed that the Manhunters not only still existed, but had infiltrated the lives of most superheroes with their agents. hey revealed themselves when a Guardian and a member of the Zamarons tried to evolve some humans into becoming the next Guardians of the Universe, during what became known as the Millennium crisis. On this occasion, there was a massive counterstrike operation by the heroes against the Manhunters, and it seemed that the Grandmaster had finally been destroyed, along with their hidden home planet. Former Privateer Mark Shaw readopted his identity as Manhunter in the wake of the battle, seeking to redeem both the name and himself.
When the Manhunters were replaced by the Green Lantern Corps, they retreated to Biot, their homeworld in Sector 3601, an uncharted area of space incapable of sustaining organic life. Hank Henshaw, the Cyborg Superman, came to Biot and became the Manhunters' new Grandmaster. He used his mastery over machinery and Kryptonian technology to upgrade the Manhunters with organic enhancements. They seem to have rebuilt the first Central Power Battery originally destroyed by Hal Jordan while he was possessed by Parallax.
The Manhunters have recently resurfaced as part of the Sinestro Corps. They carry miniature yellow Power Batteries inside them which are used by the Sinestro Corps members to charge their yellow power rings.
In Green Lantern: Secret Origin storylines, it is revealed that the Manhunters suffered a programming glitch that caused them to wipe out all life in space sector 666, believing it to be evil. It was this event that gave rise to the Five Inversions, the only survivors of the massacre, who vowed to make the Guardians pay for what their creations had done. It was later eventually revealed that the programming glitch the Manhunters suffered was caused by Krona to prove to the other Guardians of the Universe that there are flaws in an emotionless police force.