Imperial base (Imvur)

In the fourth year before the Battle of Yavin, the Galactic Empire operated a base on the planet Imvur, in the Hasmir system. Its C.O. was an officer named Viggen.

At some point, the base was raided by Commander Ryden's rebel cell, who had secret headquarters on Imvur. Viggen sought to enact vengeance by tracking down the rebels to their base and attacking them. Although the Imperials could not prevent Ryden and his men from escaping in an evac ship, the rebel headquarters were abandoned. Some time later, six cadets from an Imperial Academy and their supervisor were dispatched to the Imvur base.

Behind the scenes
The Imperial base commanded by Viggen was first mentioned in "Vulnerable Areas," a comic strip written by Martin Fisher and drawn by Bob Molesworth that was published in the seventeenth issue of the Star Wars Rebels Magazine. Although the story specified that the rebel base was located on Imvur, the location of Viggen's base was not revealed.

In "Academy Cadets," written by Fisher and illustrated by Eva Widermann for the Star Wars Rebels Magazine 18, an Imperial "base at Imvur" and its C.O. were mentioned. Since then, Martin Fisher has confirmed on Twitter that Viggen's base and the one established on Imvur were indeed the same.