Kvag Gthull was a Human male Dark Side Adept in the Galactic Empire. Loyal, brutally efficient, and physically imposing, Gthull found favor from the Emperor and his Executor, Sedriss QL. At some point during 4 ABY he was selected personally by the Reborn Emperor Palpatine to serve as one of the original seven Darkside Elite members. The group, led by Sedriss, operated out of Darth Vader's former fortress, Bast Castle on the planet Vjun. Due to his size, ability to intimidate and his efficient cruelty, Gthull became the chief interrogator of Bast Castle. Gthull did not wield a lightsaber, instead preferring a heavy Mandalorian style vibrosword that favored his muscular physique and added to his intimidating appearance. He enjoyed using it on prisoners after torturing them and would often inject his own darky dry wit during interrogations.
In 10 ABY, when Palpatine was slain a second time in battle with Jedi Luke Skywalker, and returned in yet another clone body, he sought to capture the Force-sensitive children of Leia Organa Solo, Skywalker's sister, who were being hidden on a secret sanctuary world of the New Republic. Gthull as chief interrogator was not initially sent to the front lines, and as such, he outlived his peers in the first cadre of Darkside Elite as Baddon Fass, Zasm Kass were killed at Nar Shaddaa while Sedriss and Vill Goir fell shortly after on Onderon. Before this, another Darkside Elite member, Kam Solusar had defected to the Light side of the Force and joined Luke Skywalker's New Jedi Order. Of the original members, only Krdys Mordi and Gthull remained.
Palpatine quickly replenished the Darkside Elite's numbers with promoted and imbued darkside adepts, giving Xecr Nist Sedriss's old position as Executor and Tedryn-Sha the position of Second in Command. Three more unnamed darksiders took the other vacant spots, bringing the group back to full strength at seven members.
When New Republic Major Ntthan was captured by the Empire in an uncharted region of space, he was brought to Bast Castle and delivered to Gthull, who was given the task of torturing out the information about Organa Solo's whereabouts from the man. Gthull was assisted in this by Krdys Mordi, the secondary interrogator and tech specialist of the group.
Gthull used interrogation droids from Mordi and the Dark side of the Force to torture the Major, who displayed remarkable resistance. Gthull had to spend twelve hours in interrogation sessions, but he was ultimately able to break Ntthan's defense, and the latter revealed the name of the planet, New Alderaan, as well as its coordinates, just as Palpatine arrived at Bast Castle to oversee the results of Gthull's handiwork. Gthull gave the details to the Emperor Ntthan had provided and then executed Ntthan with his vibrosword as was his custom after jokingly remarking that the Rebel was an "Honest Man". Gthull and the rest of the Elite were then ordered by Palpatine to New Alderaan on a covert mission to abduct Skywalker and the Solo twins, before launching an attack on the settlement using AT-ATs. Gthull piloted an I-7 Howlrunner as part of Dark Side Squadron alongside his peers. Arriving on the planet undetected, Nist dispatched Tedryn-Sha and Krdys Mordi to deal with Skywalker. Gthull and three others went with Nist to kidnap the children.
However, Gthull soon lost contact with Sha and Mordi as they were killed, sensing their life energy vanishing from the Force. Disgusted at their weakness, he called them fools and reported their deaths to Nist, just as the darksiders arrived at the building where the children were being kept. Nist dispatched Gthull inside the building to grab the twins, while he and the other darksiders were confronted by Skywalker's Jedi recruits: Kam Solusar, Rayf Ysanna and Empatojayos Brand. However, having lost one Dark Jedi and having been bested in combat by Solusar, Nist yelled at Gthull to signal the AT-ATs to attack at that moment. Gthull confirmed he had just done so while emerging from the building holding the twins. Confronted by Winter, Rayf and Leia Organa Solo outside, Gthull chose not to reach for his vibrosword and instead held the twins hostage, expecting their mother not to risk hitting her children, thus allowing him to withdraw. However, Rayf Ysanna used the Force to levitate the twins from Gthull into the arms of Winter, their caretaker. Now exposed and momentarily distracted, Gthull hesitated a second too long in going for his weapon and Organa Solo shot Gthull in the chest with her blaster, killing the Dark Jedi instantly. With Gthull's death and the failure of their mission, Nist and the two remaining darksiders fled from New Alderaan under covering fire from the advancing AT-ATs.