In the Himuro rituals, masks play a very important role. One of them is called the Blinding Mask, which is used in the Demon Tag Ritual. Two stakes pierce through the eyes of the Blinding Mask. It is used in the Demon Tag Ritual as a blindfold.
The legend goes, "While the demon is blindfolded, enter the rope altar and perform the ritual." This might mean that the Blinding Mask is a key to the rope altar.