Defeat Gabriel Celeste at the end of the Cave Of The Seven Stars bonus dungeon. 
Then, use the elevator in En II to go down to the Old Road to Sanctuary. Speak 
to Barago to go to the entrance of a new bonus dungeon. In the Wandering 
dungeon, you will not know the enemies that will appear in battles, and the 
floors are generated randomly. There are no save or restoration spots. Defeat 
the Ethereal Queen on Floor 20 to continue playing the dungeon endlessly.