"Due to certain restrictions, we can't leave this place for the time being. Amongst us, only my sister can leave this place," the Emperor informed Gabriel who was curious about what the Emperor wanted from him.
Although the Emperor didn't go in detail, his words made Gabriel realize a lot of things. The reason this world remained unknown to the main universe was because people from this world couldn't leave this world.
He looked at Ezekiel, thinking about something. If they couldn't leave this world, then how did they meet Ezekiel? It couldn't be that woman who met him, since she could only live for two hundred years at most. That meant the person who brought Ezekiel back was someone else.
Was there another person who could leave this world in that era? For the time being, it didn't matter. What mattered was what the Emperor wanted in exchange for a few years of his life.