Successfully complete the "Jewel hunter" quests (The Lost Princess, The Flame of Hope, Drowned Dreams, The Looking-Glass Tower, The Lucky Clover, Cosmo, Two Pearls, Alexandrite, Flourite, and Tear Drop Crystal). When those events are finished, go to the bar in Domina on Salamander day. You should see a Shadow Zero standing near the counter. Talk to him to play the game.