Qaga Lok, a descendant of the ancient Jedi Knight and Sith Lord Ulic Qel-Droma, became a mercenary during the Galactic Empire era. Displaying Force sensitivity, he worked for Imperial Intelligence but was arrested after killing Imperial officers. Facing the death sentence, he was taken into custody by the Imperial Inquisitorius and presented before Emperor Palpatine. The Emperor offered Lok a duel—win and be free, lose and swear loyalty, or face death. Lok swiftly lost the duel and pledged allegiance, but Palpatine continued the attack until Lok begged for mercy. Renamed Sedriss QL, he underwent training in both lightsaber combat and the Dark Side of the Force under Palpatine, although unaware of his actual role in the Empire.

Following Palpatine's death in 4 ABY at the Battle of Endor, Sedriss fled to the Deep Core world of Byss along with other Dark Side Adepts. As the Empire descended into chaos, Sedriss took charge of defending Byss and the Emperor's clone labs. In 9 ABY, when Palpatine was reborn in a new clone body, he rewarded Sedriss with the title of Executor, giving him full command of the Imperial Military. The Emperor entrusted Sedriss with securing the clone labs and preventing loyal Imperial agencies from seizing control in the event of his death. Additionally, Sedriss was tasked with Operation Shadow Hand, launching strikes to bring non-Imperial worlds under Imperial rule, while also leading the Dark Side Elite, a group of seven warriors empowered by Palpatine with the dark side of the Force, with Sedriss himself having slightly more autonomy than the others.

After Palpatine's death yet again at the Battle of Pinnacle Base in 10 ABY, Sedriss now initiated Operation Shadow Hand, launching strikes on the Core Worlds. Remaining loyal to the dead Emperor, Sedriss prepared for Palpatine's rebirth on Vjun. Despite the defection of Dark Side Elite member Kam Solusar to the New Republic, Sedriss continued his work with General Maximilian Veers leading ground forces. In Operation Shadow Hand, Sedriss targeted the planet Balmorra. Contacting Governor Hinch Beltane, Sedriss demanded surrender, with the governor refusing. In the consequent battle, Balmorra's forces overwhelmed Imperial troops, and Veers was killed. Conceding defeat, Sedriss negotiated with Beltane, agreeing to spare Balmorra in exchange for the war droids and designers. He however vowed eventual destruction once these terms were met.

Rumors began to circulate among the Dark Side Adepts that Palpatine might not return in another clone body, leading some to suggest Sedriss's execution for his failure at Balmorra. Sedriss discovered the destruction of most clone bodies on Byss by traitorous adepts. Engaging in a duel, Sedriss swiftly defeated and killed both adepts. Palpatine, appearing in a new clone body, tasked Sedriss with capturing the pregnant Jedi Leia Organa Solo before she gave birth and ordered the entire Imperial military against Jedi Knight Luke Skywalker. Despite sending Dark Side Elite members to capture Organa Solo, they failed, and the Emperor suggested executing Sedriss. To redeem himself, Sedriss proposed capturing Skywalker and Kam Solusar on Ossus. Palpatine approved, offering great power for success and warning of dire consequences for failure.

Arriving on Ossus aboard the Imperial hunter Scourge One, Sedriss deployed darktroopers to engage the Ysanna and weaken Skywalker. Observing the battle, he later attacked alongside Goir, utilizing the Force and engaging in lightsaber combat. However, Goir was killed, and Sedriss, disarmed and against a tree, took a Ysanna hostage. Threatening to turn her to the dark side, Sedriss intended to bring her and Solusar back to Byss. The ancient Jedi Master Ood Bnar intervened, holding Sedriss with tendrils. In a final desperate attempt, Sedriss summoned energy from the planet's atmosphere, attacking Bnar with lightning. Despite momentarily holding onto the hostage, Skywalker freed her, and Bnar used the planet's energy to obliterate both Sedriss and himself, ending the Executor for good.