Successfully complete all quests to unlock the hidden "Truth in Darkness" quest at the board at the Guild. Complete the "Truth in Darkness" quest, then create the Shadow Gem by using the item synthesis on Ruby Prism (obtained from item synthesis on Aroma Material, Brilliant Stone, Dunkelhite and Dragon Tongue) and Broken Gem (obtained during the "Truth in Darkness" quest). Then defeat the final Boss, Uroboros.