The tree at the far north of the Cloudy Stronghold has disappeared, opening the way to the Forgotten Capital. However, most of the place has been sealed off by a giant door. Instead, head west past the Geomagnetic Pole, and you will find a passage that leads you back into an unexplored area of the Echoing Library. Explore the area and touch the throne, situated behind the door to the Cradle Guardian's room, then continue to find the other passages. You will be warped back to unexplored regions of the final floors of the Golden Lair, Misty Ravine, and Lush Woodlands, touching each of the thrones on the way. Once all four thrones have been activated, the rest of the Forgotten Capital is opened up for exploration.