At the opening of the 2018 movie, The Nun, the old nun gives the young nun "a cross key", saying you know what you must do. Then the young nun commits suicide by hanging herself. What's the key for? What was the older nun trying to do in the darkroom?
The key is later taken from the young nun (Victoria)'s body and used to retrieve the vial with the blood of Christ in it. The blood is then used by the main characters in the ritual to attempt to exorcise/banish the demon and seal the rift.
This is presumably what the nuns at the beginning were also planning on doing, but they were interrupted and killed before they could get to the blood.