need help about cave of nakisawame pls

[traungo]

I just clear honomusubi cave and head to cave of nakisawame but this dun is block , i can't come this dun anyone know how it happen pls tell me what 1 miss to unlock this dun ???

[Otonashi]

Is your Purity Magatama still equipped? You need it equipped to enter.

[traungo]

oh i dont know about that so i change another magatama . Thx u so much