Note: This game is also titled Iris No Atelier: Grand Fantasm.
Successfully complete all quests to unlock the final quest, "Truth In Darkness", on the board at the Guild. Successfully complete the "Truth In Darkness" quest, then create the Shadow Gem by using the item synthesis on the Ruby Prism (from another item synthesis on Aroma Material, Brilliant Stone, Dunkelhite, and Dragon Tongue) and Broken Gem (which you got from the "Truth In Darkness" quest). Then, fight the final Boss, Uroboros, and defeat him to view the "True" ending.
Successfully complete the game and save when prompted. Return to the title screen to access the "Extra" option. Select it to view various lists, listen to music, and hear the Japanese voice acting.