When the twins reached adulthood, they requested their father's permission to lead an attack to the test of strength of the galactic powers of the greater galaxy. Valkorion approved their venture, but decreed that only Thexan would lead the Eternal Fleet. Arcann disobeyed and joined his brother anyway. Even as they led the Empire's armies in driving back the Galactic Republic and the Sith Empire. Arcann lost his arm and received heavy scarring to his face during an attack on the Sith world of Korriban, and gained a cybernetic arm and mask covering the left half of his face.

Arcann became furious when, after he and Thexan presented the lightsabers of the fallen Jedi and Sith as trophies from the battle to their father, Valkorion turned away in silence, and overcome by the dark side of the Force, Arcann attempted to attack the Emperor. Thexan, in an attempt to prevent his father's death, pulled his brother back. Arcann turned his attention to Thexan and struck out at him, but their brief duel ended when Arcann struck his brother down with a cut across the stomach, and the glint of the dark side faded from his eyes while he cradled Thexan. After his brother's passing, Valkorion instructed Arcann, now his only son, to come with him.