The Manhunters were robots created by the Guardians of the Universe as a force to maintain order in the universe.
The Guardians created the Manhunters, thinking that robots would make good peacekeepers. But they soon learned that they were flawed, owing to the inability of their programming to distinguish the subtle gradations between good and evil. Rather than decommission the robots entirely, the Guardians chose to reprogram them for lesser duties—hunting, tracking, and guarding. They didn't seem to object, at least, not out loud when the Guardians subsequently replaced the Manhunters with the Green Lantern Corps.
Over the years, the Manhunters planned their revenge against the Guardians and their Corps. Working in the Adjuras system, with the help of Kanjar-Ro, framed John Stewart, a Green Lantern, with the destruction of the Adjuras 4. They stained the Corps' reputation by making John look like a disgrace. They hoped that the Guardians would come to John's defense, while the Manhunters would lead an attack on Oa. They bypassed the defenses and prepared to take the Lanterns' power source. The Justice League and the Corps worked together to stop them. But the Manhunter leader dove into the power source, shedding his metallic form and growing to immense size. He began absorbing all of the Lantern's power. But John chanted the Lantern oath, absorbing the Manhunter into his ring and then expelled him out.
In the DC comics, the Manhunters gradually became more obsessed with carrying out their mantra than maintaining order. They eventually rebelled against the Guardians following the infamous Massacre of Space Sector 666, in which the robots annihilated nearly every living being within the sector, and were exiled. As a result, the Guardians realized that the most effective means of ensuring order throughout the universe would be through beings that felt emotion, thus creating the Green Lantern Corps. Over time, the Manhunters began to build up their forces and recruit others into a "Manhunter cult".
Recently, the Manhunters allied with Sinestro during the Sinestro Corps War.