Near the top of Tartarus, you most likely have encountered a locked Monad Door preventing you from progressing. Here is how you can unlock the final Monad Door on floor 255 in Persona 3 Reload.
How to unlock the Monad Door on floor 255 in Persona 3 Reload
Players can unlock the final Monad Door on floor 255 by completing Elizabeth’s 100th request, “Bring me a Bloody Button.” The quest requires you to find her a Bloody Button, which you can acquire by defeating the Reaper in Tartarus. Wait on any floor of Tartarus for approximately 5 minutes, and the Reaper will appear. Defeat the Reaper for the Bloody Button and return it to Elizabeth to complete the quest.
Once you bring Elizabeth the Bloody Button, you will unlock request 101, “Take out the ultimate adversary.” After accepting request 101, you must remove all members from your party and head to the Monad Door alone. The door will now open for you, putting you up against a challenging boss fight.
How to unlock the Bring me a Bloody Button request
The Bring me a Bloody Button request becomes available after you finish all Elizabeth’s other requests and have taken Elizabeth to your room during a date. Continue asking Elizabeth out on romantic dates until you have the dialogue option to bring her to your room, officially unlocking the Bring me a Bloody Button request.
Take out the ultimate adversary request rewards
Request 101, Take out the ultimate adversary, pits you up against one of the most challenging foes in the game — Elizabeth herself. The main reason why this fight is so brutal is because you have zero other members in your party and must take her down by yourself. Remember to use buffs on yourself while debuffs on Elizabeth to weaken her and give you a better chance of winning.
After defeating Elizabeth, you will finally complete the Elizabeth’s last request in the game. You will be rewarded with an Omnipotent Orb for your troubles, a rare accessory that nullifies all attacks (magical and physical) except Almighty.