Before its devastation, Callos was a jungle planet with a sweltering environment, which was home to many species of tree and beast. The Imperial bombardment destroyed much of the planet's lush surface, and sent the formerly fertile environment into a permanent winter that killed anything that survived the initial assault.
During the Clone Wars, Callos remained a neutral planet, rebuffing representatives from both the Galactic Republic and the Confederacy of Independent Systems to support their causes. However, Callos was the site of a black market operation run by outlaw tech Clawfish. In around 20 BBY, Jedi Master Kit Fisto and Padawan Ahsoka Tano went on a mission to Callos to prevent the Venomator, a highly advanced starfighter designed by Geonosian engineer Bogg, from falling into the wrong hands.
In 3 BBY, Imperial forces under the command of Darth Vader attacked Callos, after discovering evidence that the Callosians were developing a new offensive technology intended to oppose those who were spreading the New Order. During the bombardment, Vader ordered his personal fighter wing, Black Eight Squadron, to destroy the Callos planetary reactor. Juno Eclipse, leader of the bombing squad, asked Darth Vader if there was an alternative to outright genocide. Vader replied that striking the planetary reactor would suffice. The destruction of this reactor unit started a catastrophic chain reaction, which destroyed the ecosystem of the entire planet. Though the atmosphere remained breathable, incessant ice storms and sub-freezing temperatures prevented any further permanent habitation.
The reclusive Callosians developed a sophisticated culture, centered on a lifestyle of scholarship and philosophy. At various times throughout history, small groups of Callosians left their homeworld to learn about the greater galaxy, and always returned to share their knowledge with their society. Those who were away from the planet at the time of the bombardment defied logic and returned to their devastated homeworld, presumably to die. Imperial surveys that returned later found no life signs on the planet's surface and no evidence of survivor enclaves.
- "Under the Hammer"—Star Wars Comic 6
- The Force Unleashed novel (Mentioned only)
- Star Wars: The Force Unleashed comic (First appearance) (Appears in flashback(s))
- Star Wars: The Force Unleashed video game (Mentioned only)
- The Force Unleashed II novel (Mentioned only)
- The Force Unleashed Campaign Guide
- The Complete Star Wars Encyclopedia
- The Essential Guide to Warfare