The Imperial invasion known as the Siege of Lasan, alternatively referred to as the Fall of Lasan, the cleansing of Lasan, or the Lasan genocide, was masterminded by Alexsandr Kallus and resulted in the near annihilation of the Lasat species. In the early years of the Empire's reign, the Imperial Military violently put down a revolt on the Lasat's native planet, Lasan. This brutal conflict decimated the Lasat population and prompted the Imperial Senate to outlaw the use of T-7 ion disruptor rifles. While some Lasat managed to survive, such as Garazeb Orrelios, who later became a rebel, others, including Gron and Chava, found refuge on the mystical Lasat world of Lira San.
The Lasat race originated from the isolated world of Lira San, situated in Wild Space. At some point in the distant past, a significant number of Lasat established a colony on the planet Lasan, eventually severing ties with their kin on Lira San. The Lasat who settled on Lasan established a royal family and came to consider Lasan as their true homeworld. Following the conclusion of the Clone Wars, the Galactic Republic was replaced by the Galactic Empire. When the Lasat challenged the authority of the Empire, Imperial forces launched an invasion of their adopted world, thus initiating the Siege of Lasan.
In the wake of an uprising on the homeworld of the Lasat, Lasan, Agent Alexsandr Kallus of the Imperial Security Bureau was assigned the task of crushing the rebellion in 7 BBY. Though he did not initially propose their use, Imperial troops under Kallus deployed T-7 ion disruptor rifles against Lasat civilians as a demonstration of Imperial power. After defeating a member of the Lasan High Honor Guard, the guard presented Kallus with his J-19 bo-rifle in accordance with the Boosahn Keeraw, the Lasat code of the warrior. A number of Wookiee warriors, armed with weapons crafted from the wroshyr trees of their native Kashyyyk, also participated in the conflict, sacrificing their lives in an effort to prevent the massacres on Lasan. The intervention of the Wookiees was crucial in saving some Lasat lives.
Among the Lasat who survived the onslaught were Garazeb Orrelios, a captain within the Lasan High Honor Guard, and his grandmother. During the battle, Orrelios was entrusted with the protection of the royal family and the Lasat people. However, he was rendered unconscious by an explosion. Upon regaining consciousness, Orrelios discovered widespread destruction and held himself responsible for failing to fulfill his duty. The experiences of the Fall deeply affected Orrelios, and he developed a strong animosity towards the Empire. Those Lasat who were captured by the Empire were sent to prison camps.
The Siege of Lasan culminated in the devastation of the planet and the near-extinction of the Lasat species. In response to the brutality of the event, the Imperial Senate prohibited the use of T-7 disruptors, leading to their recall and destruction. Taris's senator Tynnra Pamlo witnessed the evidence of this atrocity. Nevertheless, copies of the T-7 schematics avoided deletion, and private arms manufacturers continued to produce them, with these weapons appearing on the black market. Following what he perceived as his failure on Lasan, Orrelios felt adrift until he encountered the former Jedi Kanan Jarrus, who provided him with a new purpose as a member of the Spectres crew, which evolved into a surrogate family.
The Empire secretly plotted to acquire a stock of surviving T-7 rifles with the intention of resuming their production on the planet Lothal, where the Spectres were operating. This plan was thwarted by the Spectres and R2-D2, who destroyed half of the ion disruptors and sold the remainder to the crime lord Cikatro Vizago. During the skirmish, Kallus personally engaged Orrelios in combat, taunting the Lasat about his role in the fall of Lasan.
Three years prior to the Battle of Yavin, the Spectres assisted two other Lasat refugees, the former Honor Guard Gron and the mystic Chava, in rediscovering the planet Lira San. During their journey, Gron argued that Lasan had not been destroyed, but rather transformed to fulfill an ancient Lasat prophecy concerning Lira San. This prophecy foretold that Lira San could only be found if a "child of Lasan" rescued the fool and the warrior from destruction. Following the rediscovery of Lira San, the rebels designated the planet as a sanctuary for other Lasat refugees.
Later, the fall of Lasan became a topic of discussion between Orrelios and Agent Kallus. The two adversaries found themselves stranded on the ice moon of Bahryn after an Imperial ambush above Geonosis. After Kallus recounted how he had obtained his bo-rifle from a fallen Lasat warrior, he confided that he had not anticipated the Empire unleashing a massacre on Lasan and emphasized that it was not a personal matter. He also admitted that, despite having taken credit for the use of the T-7 ion disruptor rifles, the decision to use them was not originally his. Orrelios assured his adversary that he had already come to terms with the past. As a gesture of reconciliation, he presented Kallus with a Bahryn meteorite as a parting gift.
As a result of his time on Bahryn with Orrelios, Kallus defected to the rebellion, serving as a new Fulcrum agent. However, while Kallus operated as an undercover spy for a period, he was eventually discovered by Grand Admiral Thrawn and fully joined the rebellion after being rescued during the final stages of the Battle of Atollon. Following the Galactic Civil War, Orrelios guided Kallus through the secret hyperspace route to Lira San, where Kallus witnessed that he had not destroyed the Lasat people and was welcomed to join them.
Imperial defector Yrica Quell considered the difference between Alderaan's destruction, as well as genocides on Lasan and Dhen-Moh, and Operation: Cinder to be the distinction between cold ruthlessness and outright cruelty.
The Siege of Lasan was initially alluded to in the television movie Star Wars Rebels: Spark of Rebellion, which premiered on Disney XD on September 26, 2014. It was first explicitly mentioned in the first episode of Star Wars Rebels, "Droids in Distress," which aired on October 3, 2014.