Aeternus made the demon gods arrive before the Tyrants could see the image of the abyss through the barrier and prepare for it. This ensured that the Tyrants were caught off guard, and the possibility of total realm destruction became a certainty.
But Aeternus couldn't take full credit for this achievement. He muttered inwardly as he saw the events happening in the Tyrant realm through Helios' eyes, "I am sure this is the main reason the realm lord offered the demon gods a deal. He might have made it look like he didn't want the demons to betray the realm, but I am willing to bet it was to achieve a swift victory in the era of conquest."