As you approach the Great Labyrinth, you are stunned to see a massive stone structure standing in your path. In order to enter the halls, it appears you'll need a Great Labyrinth Key.
You walk up to the curious doorway, and insert the key into a small hole in the wall that matches the pattern. Something behind the walls begins to hum, and a vicious grating sound erupts from the doorway! Your key is crushed as the stone shifts around, and a gust of stale air rushes out of the new entrance. For today at least, the way to the labyrinth is open.
You decide to wait for now. From what you can tell, each key gets only a single use, as the locking mechanism on the massive structure is designed to destroy any trace of an adventurer's entrance.