So the requirements for the inf castle:
Killed 200 demons
Mastered the breathing of the player choice
*Blood from lower demons and uper demons (in order to boost grind and map explore) *
Had at least met a Hashira
Unlock Mark
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To start the ritual 4 people must be gathered around, each player need all the requirements, after that each players uses the items. Doors will stand right beneath them dropping them down to the inf castle, Each player will fall differently based on the breathing they choose, from example people who choose wind breathing wil drop down a bit unserious, people who chooses zenitsu's breathing will fall all serious will eyes closed prepared for everything etc etc.
Stage 1
Each demon slayer will drop in a different level with many denons, they won't be together resulting of each slayer to defeat many demons
Stage 2
After each denon slayer defeat their demons a door will open under them, dropping them in a room where they gave to fight the 2 strong upper moons (sorry but I don't remember their names)
Stage 3
After succeeding a cutscene will play where the demon with the sword will drop from the ceiling,
Stage 4
After the players defeated that boss it's time for the final boss, Muzan, Muzan will have 2 stages resulting in a intense fight where demons corps need to cooperate.
Pls feel free to remove or add anything stages you want, even edit them. But at least if you find my idea interesting pls don't hesitate to contact me
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**Rewards**
After the fight they will be a cutscene where everyone uses their breathing and kill muzan, feel free to add any rewards you guys want
(Pls forgive me if I made grammar or vocabulary mistakes)