Kalast was an Imperial Moff who betrayed the Galactic Empire during the Galactic Civil War.
Using the Bothan Spynet to gain top priority secret Imperial codes for the Death Star, he slipped the data into the Rebel Alliance's hands. He later met up with the Alliance in a secret installation on Thyferra. Darth Vader, intent on discovering the identity of the traitor, tracked him to Thyferra and led an attack force on the planet. Kalast was able to escape because the Imperial fleet was detected using a sensor system.
In a prototype Imperial II-class Star Destroyer named the Avenger, he remained in his retreat above Atzerri. His Star Destroyer was also painted with red markings, showing Rebellion affiliation. The Empire tracked him down there, and the Imperial Navy moved into the system. Kalast jumped into hyperspace and escaped. After the siege of his Atzerri retreat, he fled to a number of worlds, until any further retreat on his part was stopped by an Immobilizer 418 cruiser. His flagship was severely damaged, and he surrendered.
He was interrogated by Emperor Palpatine himself and, under torture, he revealed that he gave the top secret plans to the Bothans. After his usefulness was over, Kalast was executed for treason.