This feature is a secret dungeon that can only be unlocked on your second playthrough when you have saved Shiba and completed the Ortion Cavern (East) side quest. When you have accomplished this go to Sheridan and talk to the two men in the northwest part of town and then Shiba and you will be taken to the dungeon.
Completion Bonuses: To unlock 'Very Hard' and 'Unknown' difficulty setting and the 'Game Record' and 'Sound Test' options at the Main menu you must successfully complete the game or start a new game with the cleared game file to have access to the Grade Shop.
Asch Stays in your Party Permanently: When there is no loading occurring in the OverWorld open the disc tray and remove the game disc. Then shut the disc tray without the game disc in it and you will be able to go anywhere in the OverWorld. Once you get to the desired spot open the disc tray and put the game disc back in. You will then be able to get Asch on the Albiorl. Doing this though will mean you lose Luke.