In the mirror universe, the Genesis Weapon was a powerful weapon developed by Carol Marcus and her son David Marcus. It had the ability to destroy entire planets by reducing them to subatomic particles. If so desired, the weapon could also remake the targeted world as a perfectly terraformed planet, ready for immediate colonization.
The weapon was aimed at Ceti Alpha V during a struggle between Spock and fellow Vulcan T'Pau. T'Pau had been declared an enemy of the empire for assassinating Emperor Danaher; T'Pau, along with Magda Kovacs and others loyal to their cause, seized the Imperial research base Regula I where the weapon was being developed. During the struggle in the weapon's chamber, Genesis was activated; however, Magda Kovacs fell into its beam, reducing her to atoms and causing a feedback which then destroyed the weapon. (Decipher RPG module: Through a Glass, Darkly)