The certain sacrifice
BEWARE SPOILERS
It is implied that, when the mob is irate and intend to sacrifice Señor de Nobile, that another sacrifice has taken place.
Not one of blood, but rather other thing. If you all remember, La Walkyria is a virgin, and remains so ostensibly throughout the film. However, why does Edmundo thank her for something she gave freely -- as he clearly says in the scene preceding the resolution -- after both of them have absconded for several minutes behind a curtain?
My take is that Leticia, la Walkyria, sacrifices her virginity so that everyone can leave. Of course, it's only an interpretation, but if you look closely at the scene, you may find it might be one of the myriad intentions in Bunuel's plot.
And it's, evidently, open to any interpretation.
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Witches... All of them witches... unspeakable, unspeakable!