Successfully complete the game. Load your cleared saved game file. Enter the 
left door after going up the stairs after the final save point. Defeat the 
Undying Dragon to get the Darkness Ring. Return to the Coliseum in Tatroi. Go to 
the end of the left hallway on the first floor. Use the Darkness Ring on the 
glyph in the wall to access the Cave Of The Seven Stars bonus dungeon.