You’ll need four masks to face off against Lady Dimitrescu. You get one by following a linear sequence of events, but the others are a bit trickier. One mask you’ll need is called the Mask of Pleasure, and Resident Evil Village uses some classic tricks to make it so you can abscond with it.
After you get the Dimitrescu key, go back to where you unlocked the Prioress door. There’s a Dimitrescu key room through there with the Mask of Pleasure sitting in it. After taking the mask, a gate will block your exit. You can’t go back through this room’s entrance. But before you set off to leave, be sure to grab the silver ring treasure in a drawer.
Once in the next room, the second daughter will attack you. Immediately move the dresser on the wall, which will reveal that the wall is damaged. Throw a pipe bomb at the damaged wall, letting in cold air.
Resident Evil Village — leaving with the Mask of Pleasure
As the Dimitrescu daughters are made of flies, this weakens them, allowing you to shoot them. Once she’s dead, grab the animal skull off the wall and examine it. Taking a look at the back will reveal a bolt securing the skull to the rest. Remove the bolt and then go back to the previous room. Approach the spot where you got the mask and use the skull from the menu. This will put it in place and raise the gate, allowing you to get the hell out of there.
As for why an animal skull would have similar marks on the back as a mask, I don’t really know. Nor do I know why Dimitrescu locks the door to a section of the castle with four masks instead of just using keys like a normal person. I guess we’ll have to chalk it up to nine-foot-tall vampire lady logic.