The Planet Killer represented a massive, globe-shaped superweapon engineered for the express purpose of annihilating any planet upon which it was triggered. During the Clone Wars era, specifically in 22 BBY, the creator of this Planet Killer sought to broker a deal, offering the devastating device to the Confederacy of Independent Systems. Count Dooku, the leader at the helm of the Separatist cause, orchestrated a test run of the Planet Killer on a planet brimming with various lifeforms. However, unbeknownst to the Separatists, Jedi Master Mace Windu intervened, deactivating the Planet Killer before its activation sequence could complete. Consequently, Dooku, deeming the Planet Killer a failure based on this incomplete test, ordered the imprisonment of the superweapon's architect.
The Planet Killer was a sizable, spherical superweapon and bomb designed with the theoretical capacity to completely eradicate a planet upon its detonation. The device was designed to generate an intense flash of light and a colossal explosion, resulting in the planet's utter destruction. However, the outer shell of the superweapon lacked resistance to lightsabers, and the device could be rendered inert by removing its detonator.
Positioned atop a flat base, the Planet Killer possessed substantial height and width, measuring several meters in both dimensions. Four cone-shaped structures protruded from the weapon's central region, each terminating in a smaller sphere. The Planet Killer's coloration consisted of orange, gray, and white hues, with yellow light emanating from narrow slits that encircled the weapon's circumference.
A non-Human inventor conceived the Planet Killer during the latter years of the Galactic Republic. In the year 22 BBY, specifically five months after the Battle of Geonosis which marked the commencement of the Clone Wars, the Planet Killer's inventor sought to sell the device to Count Dooku, the leading figure of the Confederacy of Independent Systems. To evaluate the weapon's destructive potential, Dooku arranged for a demonstration of its power on Omphalos, a world abundant with diverse forms of life. Dooku deployed the Planet Killer onto the planet's surface, assigning several B1-series battle droids and a pair of B2 super battle droids to guard the superweapon. Meanwhile, Dooku and the weapon's creator observed the proceedings from a starship positioned in orbit, awaiting the weapon's activation.
However, the Republic became aware of the Separatists' intentions to obliterate the planet and dispatched Jedi Master Mace Windu alongside a contingent of clone troopers to thwart the Planet Killer's detonation. Despite the downing of his LAAT/i gunship and the deaths of all his troopers, Windu successfully fought his way through the Separatist battle droids guarding the Planet Killer. Employing his lightsaber, Windu breached the Planet Killer's outer shell and extracted its detonator just before activation, effectively disabling the superweapon. Unaware that the Republic had disrupted the test, Dooku concluded that the Planet Killer was indeed non-functional due to the planet's failure to explode, and ordered the weapon's creator to be taken prisoner as a consequence. With the Planet Killer deemed a failure, Dooku shifted his focus towards alternative avenues for developing an "Ultimate Weapon."
The Planet Killer's sole appearance in Star Wars Legends occurred within the pages of the comic titled Run Mace Run, featured in Star Wars: Clone Wars Adventures Volume 2 released on November 17, 2004.