Help finding a ring

[Atrix]

Hello! I am new to the game so far have beaten Mycelium boss, however trying to locate some secret areas in here it seems a lot of people weild a green ring up to this point probably a stamina ring or something and I don't seem to be able to find it, could anyone know how to get it? Also there is a lonely room up top the boss room at mycelium that I cant find uow to enter, saw it in another players map.