In Spiny Mole's tunnel in Nova, notice the large 'X' on the wall. Return there after visiting Shadra for the first time and you will gain entrance to the secret dungeon, Glissina Cave which contains powerful enemies and strong items. You cannot enter here if you have already fought Shadra's first form, however.

 

 

 

Prizes from using tag mode

Tag mode lets you exchange Amigos, when you get to a certain amount you get a predetermined prize. These prizes are all extremely useful for the main game, they range from equipment that raises stats considerably to useful replacement party members to a decent ability that can do huge damage, especially at the beginning of the game. *This requires two DS, two copies of Magical Starsign* start tag mode, choose a file and exchange in order to get an amigo. For additional Amigos start new game save on an empty file>tagmode>exchange>repeat. Prizes are list below