The Limbo Prison!

"It's only possible to enter but not to exit. What does this mean?"

Merlin and Setoh's faces turned ashen. They did not wish to be trapped in the Flare Region world for countless years or they would be unable to visit the Giant Tribe to help the Spell Caster civilization. The consequences would be unthinkable.

Flarite looked up with a bitter smile. "You've heard earlier that the entire Flare Region world is a prison to detain the Vestigial Tribe! This prison is incomparably solid. Whether from outside or inside, it's nearly impossible to exit. If a prison allows one to go out easily, what kind of prison is it? Of course, Sir Flare Emperor had said that the exception would be those ultimate existences who could sense the traces of the natural order left here. They might still be able to break free from this prison. However, from the looks of it, you're not ultimate existences, right?"