Slenn was an Imperial Vice Admiral operating under the authority of Darth Vader.
Slenn was tasked by Vader with leading a military excursion targeting the Rebel Alliance on the world of Datar. He prematurely deployed his fleet, giving the Rebels advance warning of the impending assault. This misstep would prove to have grave consequences for Slenn.
Although the Imperial troops achieved a tactical victory, eliminating all Rebel combatants, Slenn's command neglected to capture any prisoners, which represented the operation's core objective. Due to these shortcomings in his leadership, the Vice Admiral met his end at the hand of Emperor's enforcer, Darth Vader.