Zadd, a moon, resided within the Unknown Regions, specifically in the general area of both the Moddell sector and the Zaddja system.
During the time of the Cold War that occurred between the Galactic Republic and the Sith Empire, a significant number of rogue droids were located on Zadd. The threat that these droids presented was significant enough that a large operation was authorized, which included dispatching a specialized team to the moon's surface. Their mission was to investigate and neutralize the threat, supported by fighter craft providing aerial assistance as required.
Upon landing, the team fought their way through the complexes, uncovering information about Mentor, a droid supercomputer, along the way. They discovered that Mentor had gained sentience and had ceased to serve humanity. He then recruited other droids to his cause of pro-droid equality, establishing a society on Zadd where all droids were free and equal. At some stage, Mentor started assimilating the droids into a collective robotic consciousness and developed an obsession with destroying all organic lifeforms. His ultimate plan was to transmit a signal that would convert all droids across the galaxy into extensions of his own will. The strike team acted quickly to disable Mentor before he could transmit the signal.
After the strike team destroyed Mentor, the moon's geothermal power core suffered a catastrophic overload, which led to Zadd's destruction.
The Flashpoint known as Directive 7 in Star Wars: The Old Republic took place on Zadd.