 Underneath the island, there are a number of very long tunnels with monsters locked within crystals.The whole novel is about Outsiders attempting to breach the prison, undo the magic that made it, and free all the great evils imprisoned below it.While the spirit was held at bay by two Queens of the Fey Courts and their entourages working together, it was also inferred that on the island, the spirit had the power to capture Mab herself and imprison her if needed.