The Battle of Callos was a battle that occurred on the planet of Callos, and was led by Juno Eclipse and her Black Eight Squadron. After destroying the planets defenses, Darth Vader ordered that the planet is bombed continuaslly until all life is destroyed. Vader may have wanted the planet to be just an example of the Empire's power.
Juno tried to tell Vader that the defenses were destroyed and the empire won. She couldn't destroy them when there was no defenses. Vader then said that she could just destroy key areas if she felt so strongly about the other orders. Juno then realized after she destroyed the key areas that, almost no life was found. She hates speaking of it because it became a dead world.