When the flying ship came out of that world, Darian immediately activated his power to sense the surroundings, but there weren't any Risen nearby. That place was completely cut off from the outside world, and neither the Risen nor Celestia Academy would know anytime soon what had happened.
There were still at least a few weeks left before the date when, in theory, that world would close. When those who went in didn't come out, that would be the moment when those two powers would notice the anomaly and investigate, eventually finding out that the world no longer existed.
That's why these days were crucial for Darian to take advantage of. After all, the Risen had placed parts of the World Tree in the three member planets of the galaxy's inscription, certain that if Darian tried to invade the core, they'd find out and stop him.
But now, with the owner of those things right there with them, they could invade easily.
