long story short; muzan went to Kagaya and Kagaya blew himself up along with muzan in there, hashiras find out and rush there, tamayo traps him in a BDA as the hashira + tanjiro arrive there go to fight him but falls into the infinite castle, nearly all the demon slayers that were around...now ur in the infinity castle confused with ur sword out looking around while u see demon slayers falling from the top like tanjiro or zenitsu...