Reach the top of Tartarus, and successfully complete Elizabeth's request #53 to unlock the Monad Block. Monad is a special block inside Tartarus with ten floors containing level 90+ Shadows.
Successfully complete the game, and save when prompted. You can use your cleared saved game file to start another session with all progress gained from your previous play through. Your playtime, Charm, Academics, Courage, main character's level, yen, Persona compendium, max S. Link items, and completed fusion spells will be retained. You will also have the weapons, armor, and accessories that were not equipped when your last game ended. Additionally, if you completed Elizabeth's request #53 in your original game, you can get her request #55 with the new session.