Teleport to Onett and enter the library to the north of the town. Once inside, you will see two doors. Take the one to the right. The book is in the far left bookshelf of this room. Grab it and head back outside. Teleport to Tenda Village and give the book to the chief.
Once you have confirmed your name again, talk to the chief to get a Bag of Dragonite which you should have Paula hold for the time being. Save this item for a difficult boss battle later in the game. Talk to the Tenda standing next to a table with a cup on top to be spoken to by a higher power once again, similar to the Cup of Coffee at Saturn Valley. You can also talk to the Tenda to the north who will trade you items for Horns of Life. To get the item you want, count how far down on the list it is and give that number of Horns to the Tenda. For example, the Hall of Fame Bat will cost you seven Horns of Life.
Once you are rested up and have saved, talk to the strong Tenda to have him move the boulder. Enter the cave to Lumine Hall.
You'll begin Lumine Hall on the upper floor, out of two floors. To reach the Sanctuary you must navigate through both floors many times, encountering some fairly dangerous monsters in the process. The upper floor has a ton of little orange Fobbies that resemble the Foppies you fought in Belch's base. These enemies are actually very easy and you will fight up to six at a time. Just use Paula's PSI Fire spell to take them out in one turn and gain a ton of experience. This is a great way to level up if you need it.
The bottom floor has an enemy called the Uncontrollable Sphere which has a 1 in 128 chance to drop the Broken Antenna. This item can be fixed to become Jeff's best weapon, the Gaia Beam, if you care to find it. Be very careful because the Spheres are extremely hard. They explode after battle causing massive, usually mortal damage to each character. If you wish to find the item, remember that you can rest and save at Tenda Village nearby. Once you beat the boss here, you won't be able to fight the Uncontrollable Sphere again and then with only one character, so it is recommended that you take the time to get the item now. If you beat the boss, all enemies will try to get away from you, including Uncontrollable Spheres, making it easier to attack them from behind.
Begin the cave by following the path to the north and left, ignoring the first ladder you come across. Pass the hole and continue left to take another ladder down to the bottom floor. Grab the IQ Capsule to the south and then head up the ladder nearby. Follow this path to the Diadem of Kings which you should equip on Poo immediately. This item increases Poo's luck and resistance to PSI attacks and is one of his only armors in the game. Backtrack to the hole you passed earlier and drop down.
Grab the Super Bomb from the present and follow the path to a ladder. Take this ladder back up to the top yet again and you'll notice that you are back at the first ladder you passed in the beginning of the cave. Head north and this time take the northern split all the way around to a Luck Capsule and another ladder back down to the bottom floor.
Once back on the bottom floor, follow the path to a Rock Candy and farther to a Bottle of DX Water. After you grab the water backtrack to the ladder you just passed and take it to the top floor. Grab the Cup of Lifenoodles from the present next to the hole and then drop down. Follow the path around and grab the Luxury Jerky on the way to a ladder. Climb the ladder and you'll find the Sanctuary boss. Not far from the boss is the Rabbit's Foot. At this point you have all the items in the cave, so talk to the shiny spot to fight Electro Specter.
After defeating Electro Specter head down the path and drop into the hole. Record the melody with your Sound Stone and continue to the left to drop down another hole into the Lost Underworld.