To unlock Universe difficulty, complete the game on Galaxy difficulty. To unlock Chaos difficulty, complete the game on Universe difficulty.
Cave Of The Seven Stars bonus dungeon: To access the Cave Of The Seven Stars bonus dungeon, beat the game and 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.