My first theory is kind of straight forward. I believe it's referring to Yorshka, since she's Gwyndolin's sister, and also a young, pale girl in hiding. Aldritch also 'perceived the form of a young, pale girl in hiding while devouring Gwyndolin. Also I wouldn't refer to Priscilla as a 'young' girl since it's been ages since Dark Souls. My second theory, which destroys my first theory, is that it is indeed referring to Priscilla. The spell has the same name as Priscilla's Scythe, which has God killing powers. And Gwyndolin is the son of Gwyn, the First Lord of Cinder, and indeed he was a God. Or it could refer to both Priscilla and Yorshka at the same time. And that they are related. I also have a wild theory which I'd rather keep to myself.