Kromus existed as the fourth planet within the Kro'eval system. Boasting a vibrant biosphere, it was home to a sentient population that numbered 1.3 billion by the time the Clone Wars erupted.
During the Clone Wars conflict, Kromus, situated near the boundary of Separatist-controlled territories, became the location for a research installation where the Starkiller superweapon was under construction. This construction occurred at the center of a city inhabited by two million individuals. On this planet, the CIS was conducting a superlaser experiment.
However, in the archives of the Galactic Republic, data pertaining to Kromus was incorrectly exchanged with data concerning Kromol, the fifth planet in the system. As a result, the Republic operated under the false assumption that the Starkiller was being developed on an uninhabited world.
The Republic only became aware of this error when they deployed a fleet, commanded by Jedi General Darrus Jeht, with the objective to destroy the Starkiller. Jeht, convinced that there was no alternative to prevent the superweapon's escape, gave the order for the command cruiser Maelstrom to bombard the planet, thereby eliminating the secret weapon. The resulting explosion instantly annihilated the city and triggered a planetary catastrophe that swiftly extinguished all life forms on the planet.
Kromus experienced the stripping away of its atmosphere, and its crust endured extensive tectonic disturbances. The death toll exceeded one billion sentient beings.
Due to the information mix-up, the true designations of the fourth and fifth planets in the Kro'eval system remain somewhat uncertain: while the fourth planet is typically called "Kromus," and that is the name used here, Darrus Jeht later refers to it as "Kromal," possibly suggesting that this is actually the correct name.