Umak Leth served as an engineer for the Imperial forces. Notably, he held a key role as one of the primary weapons engineers assigned to the original Death Star.
Prior to the outbreak of the Clone Wars, Leth represented the Galactic Republic's Ministry of Science. In 22 BBY, he participated in a HoloNet debate, facing off against Nasdra Magrody and Bunt Dantor. During this debate, Leth posited that the mysterious weapon that was targeting Republic forces may have been equipped with cloaking devices. His calculations suggested that a power source capable of destroying entire task forces of capital ships would necessitate a vessel with a length of at least 8 kilometers. Furthermore, he theorized that a Jenth-class hypermatter core could be the reactor powering the weapon.
During the era of the Great Jedi Purge, Leth was responsible for designing the Universal Energy Cage, a device intended for the containment and transfer of Jedi prisoners. In addition, he engineered the "resonator" devices, which functioned as weapons employed to torture and subdue the Marasan population. His work on the World Devastators began as early as 0 BBY, kept under such strict secrecy that Leth only provided Grand Moff Wilhuff Tarkin with the most essential details, which Tarkin subsequently incorporated into the official field manual titled Imperial Handbook: A Commander's Guide. Following the Battle of Yavin, Bevel Lemelisk's reputation suffered, and Leth was promoted to Master of Imperial Projects in recognition of his creation of an advanced molecular furnace model.
Years afterward, Leth engineered the Galaxy Gun for the resurrected Emperor Palpatine. He also worked on the World Devastators, SD-10 battle droids, and Shadow Droids. Furthermore, he presented the cloned Emperor with the XR-85, finally bringing the tank droids to their full potential. His life came to an end when Palpatine's flagship, the Eclipse II, emerged from hyperspace and collided with the Galaxy Gun.
In the audio dramatization of Star Wars: Empire's End, the voice of Leth was provided by John Cygan. In HoloNet News — A Galaxy Divided, David W. Collins voiced the character.
There were plans for him to appear in a comic titled Boba Fett vs. The Empire, where Leth would betray the Empire, leading to Boba Fett being assigned to track him down; however, this comic project was ultimately abandoned.