"Underground Dungeon?" Leo asked, tilting his head. "Why is that under the temple?"
[This cave was originally created to seal and store one of my nine most dangerous relics,] the World Tree explained.
"Ah," Iris murmured as if she finally understood, nodding slightly. "So that's why it was hard to figure out the magic inside the barrier?"
[That's right, child. This barrier does not welcome visitors who do not bear the royal mark, and it does not allow them to leave,] the World Tree continued.
The magic of the barrier surrounding the underground cave separated the space, preventing anyone without the royal mark from entering or leaving. However, there were no fatal restrictions on the inside itself, other than one: you could not exit.
"But... since it's called a dungeon, it feels a bit creepy. There's nothing strange down there, right?" Leo asked cautiously.