The original Lunatic Legion was comprised of like-minded blue Kree who held strict beliefs on Kree purity and the dominance of blue-skinned Kree. One of the groups main objectives was to overthrow the Supreme Intelligence, and another was the death of the Kree hero Mar-Vell. The group operated on a secret base on Earth's moon, where they conspired against Mar-Vell.
It was the Lunatic Legion who turned a human being into the supervillain Nitro and dispatched him to steal Compound Thirteen. This resulted in the fateful battle between Nitro and Mar-Vell that led to Mar-Vell's eventual death.
Unable to defeat Mar-Vell, the Lunatic Legion fled back to Hala where they were summoned to an audience with the Supreme Intelligence. The Intelligence proposed an alliance with them to eliminate Mar-Vell, though the real purpose of this was to pit the Lunatic Legion into a battle they could not win against Mar-Vell. The plan worked, and the Lunatic Legion was defeated.
A new Lunatic Legion was formed in the aftermath of the first Kree-Shi'ar War and Operation: Galactic Storm. However, it was not called the Lunatic Legion at first. It began as an unnamed unit of Kree soldiers led by Admiral Galen-Kor intent on avenging the loss of the Kree Empire in the war. Earth was one of their main targets, because not only had the Avengers interfered in the war but a contingent of that group killed the Supreme Intelligence. Galen and his fellow Kree soldiers had heard the talk that the Supreme Intelligence conspired to have the Nega-Bomb kill so many Kree, but they believed it to be nothing more than Shi'ar propaganda.
Secretly allying with the Collector, Galen-Kor was able to acquire a Nega-Bomb and bring it to Earth with the plan of detonating it. As part of the bargain, Galen had to deliver the Avengers to the Collector. He kept this alliance secret from his Kree soldiers, causing Dylon-Cir to question his actions often. They were able to ambush and capture the Black Knight, Crystal, Hercules and Deathcry. Kona-Lar and Dylon-Cir signaled out Black Knight and Deathcry for torture, because Black Knight had been the one to deliver the death stroke to the Supreme Intelligence and Deathcry was from the Shi'ar Empire. However, Galen and his Kree were unable to capture the rest of the Avengers when the team arrived to save their comrades. The Kree armed the Nega-Bomb to detonate and fled the planet, returning to space to resist the Shi'ar occupation of Kree space. The Avengers were able to prevent the Nega-Bomb from killing Earth.
Galen and his Kree resistance fighters returned to Earth soon after this. Their new objective was to locate a Shi'ar ship that had come to Earth long ago and free one of its occupants. This occupant was an ancient Mephitisoid warlord, and the Kree believed they could use his pheromone ability against the Shi'ar. The Avengers interfered, and the Kree were driven away without the alien they had come for.
It was on their next return to Earth that this group began calling itself the Lunatic Legion. They discovered the Supreme Intelligence lived and was being held in a prison on Earth's moon. Galen-Kor led his Lunatic Legion on an assault that took over the prison, which they then used as their secret base. To honor the Supreme Intelligence, their plan was to unleash a weapon that would either kill the human race or turn its members into Kree. As Galen described, they would live Kree or die. The Lunatic Legion began developing the weapon covertly, gathering technology from Earth and performing cruel human experimentation. Their activities eventually got the attention of the Avengers, who moved to stop them. They hurried to acquire the final component necessary for the weapon, which was a container of the Terrigen Mists. All the while, the Supreme Intelligence did not wish them to succeed but could not say so while at its own people's mercy. The Lunatic Legion were not only willing to die to complete their mission but planned to as part of the weapon's activation. During their battle with the Avengers, the bodies of the Lunatic Legion were transformed into energy and disintegrated to fuel a weapon that the Avengers were able to prevent from working. It seemed as though the Lunatic Legion was now dead. A few survivors were brought to the Fantastic Four to be deported for trial.
Apparently, at least some if not all of the Lunatic Legion were able to survive and were somehow later reintegrated. Galen-Kor was later shown to be alive and serving as an admiral in the Kree military once again.