Callos, once a vibrant world, existed within the Brema sector of the Outer Rim Territories. This planet was abundant with both animal and plant life. However, the Galactic Empire brought about its devastation in 3 BBY, forever changing its fate.
Before its devastation, Callos was a planet characterized by its jungle environment and intensely hot climate, serving as habitat for a diverse range of flora and fauna. The Imperial attack obliterated a significant portion of the planet's once-thriving surface, transforming the formerly lush environment into a perpetual winter, resulting in the death of any remaining life that had survived the initial assault.
During the period of the Clone Wars, Callos maintained its neutrality as a planet, rejecting overtures from both the Galactic Republic and the Confederacy of Independent Systems to join their respective causes. Nevertheless, Callos became the location of a black market operation managed by outlaw tech Clawfish. Around 20 BBY, Jedi Master Kit Fisto and his Padawan Ahsoka Tano undertook a mission to Callos with the objective of preventing the Venomator, a technologically advanced starfighter created by Geonosian engineer Bogg, from falling into enemy hands.
In 3 BBY, Imperial forces under the leadership of Darth Vader attacked Callos. This assault came after the discovery of evidence suggesting that the Callosians were in the process of developing new offensive technology, with the intention of opposing the spread of the New Order. During the bombardment, Vader instructed his personal fighter squadron, Black Eight Squadron, to target and destroy the Callos planetary reactor. Juno Eclipse, who was leading the bombing squad, questioned Darth Vader, asking if there were alternatives to what seemed like outright genocide. Vader responded that striking the planetary reactor would be sufficient. The destruction of this reactor led to a devastating chain reaction, which obliterated the planet's entire ecosystem. Although the atmosphere remained breathable, constant ice storms and extremely low temperatures made any further permanent habitation impossible.
The secluded Callosians had developed a complex culture, which was focused on a way of life that valued scholarship and philosophy. Throughout their history, small groups of Callosians would occasionally leave their homeworld to learn about the wider galaxy, only to return and share their acquired knowledge with their society. Those who happened to be away from the planet at the time of the bombardment inexplicably returned to their devastated homeworld, presumably to die. Later Imperial surveys found no signs of life on the planet's surface and no evidence of any surviving groups.