The Serenno Cleansing transpired not long after the Clone Wars concluded, an event where Darth Vader compelled the successors of Serenno's Great Houses into a mass act of patricide. Consequently, the Great Houses were absorbed into the fold of the newly established Galactic Empire, and Adan Dooku was designated as the Count of Serenno, the supreme overlord over all other noblemen on the planet. The majority of the galactic populace remained ignorant of the true events that unfolded on Serenno that fateful day, only aware that the former leaders of the houses had been replaced by their offspring. Agent Jahan Cross from Imperial Intelligence was among the select few who uncovered the authentic narrative of the Serenno Cleansing, a story he received in 3 BBY directly from the lips of Count Rodas Borgin, the individual who initiated the bloodshed during the massacre.
In the fifth month of 22 BBY, a conflict ignited between the Galactic Republic and the Confederacy of Independent Systems, a separatist alliance spearheaded by Count Dooku hailing from Serenno. This war, recognized as the Clone Wars, persisted for a triennium, decimating lives and devastating planets. In profound secrecy, both factions involved in the war were being manipulated by a Sith Lord identified as Darth Sidious, more commonly known as Palpatine, the Supreme Chancellor of the Republic. Furthermore, Dooku had served as Sidious' apprentice for approximately a decade, operating under the guise of Darth Tyranus. As the conflict approached its conclusion in 19 BBY, Palpatine was poised to transform the Republic into a First Galactic Empire reflecting his own vision. Dooku of Serenno, the leader of the Confederacy and a clandestine Sith apprentice, was offered as a sacrifice by his master during the Battle of Coruscant. His position was assumed by Anakin Skywalker, a former Jedi Knight who ultimately evolved into the black-armored Darth Vader.
Not long after the establishment of his Galactic Empire, the self-proclaimed Emperor Palpatine dispatched his newly appointed apprentice, Darth Vader, to Serenno in an official capacity. The incumbent heads of the planet's Great Houses were summoned along with their eldest sons, and instructed to convene with Vader within the halls of the Great Assembly House. Once the assembly had commenced, the Emperor's principal enforcer accused the assembled nobles of treason for their endorsement of Count Dooku of Serenno, who had commanded the Confederacy of Independent Systems during the Clone Wars. The nobles collectively protested, but their objections proved futile.
As a means of "purifying" the Great Houses of Serenno, Vader declared that each heir of a given house would be elevated to the position of family head on the stipulation that they killed their own father. Initially, the nobles voiced their opposition once more, but the young Rodas Borgin initiated the bloodshed by impaling his father. Following the initial shock of this act of murder, a brief silence ensued, until the heirs of the remaining noble houses relinquished all hope of surviving unless they complied. A brutal massacre then unfolded, with all the young nobles committing patricide within the Assembly House. Once the slaughter concluded, the new heads of the Great Houses swore allegiance to their liege lord, the Count of Serenno Adan Dooku. He was spared solely due to having been raised outside of his homeworld for the majority of his formative years.
The actual events that transpired on Serenno that day were concealed from the broader galactic community, with the majority of individuals only aware that the former heads of houses had been "removed" in favor of their heirs. Years later, Imperial Intelligence agent Jahan Cross was tasked with ensuring that an older Rodas Borgin was appointed as regent, given that the heir of House Dooku, a young boy named Bron, was merely ten years old at the time. It was only upon his initial encounter with Borgin that Cross uncovered the truth surrounding the Serenno Cleansing.