It was a scaling weapon. As long as it was used to kill Doomguard, it would absorb their apocalyptic power, growing stronger with each life it took. In essence, now that Orion had unequivocally chosen his side, he needed a weapon like this to properly lead the Scourge Wardens. Conversely, the stronger the blade became, the more Doomguard he must have slain, a fact that would only deepen the reverence and awe the Scourge Wardens held for him.
"We swear our lives to you, my lord!" Eparus roared, dropping to one knee. "To conquer the Abyss, we will pay any price!"
"We swear our lives to you, my lord! To conquer the Abyss, we will pay any price!" The chant was picked up by every Warden, a thunderous oath that shook the scorched earth.
At that moment, Orion knew, they had truly become his. He didn't need to state the obvious. Their absolute loyalty was a powerful asset.
