Is The Nun Based on a True Story?
Is The Nun Based on a True Story? Let's Go Over the Facts
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The disconnect here is that Lorraine Warren never claimed to have been haunted specifically by the entity known as Valak (at least, not that I could find). But that's not to say that Valak isn't a figure that already exists in our world. In fact, mentions of the demon date back to the 17th century, in a book known as Lesser Key of Solomon. It was a "grimoire" of sorts, a guide to demonology. It's not hard to find a PDF file of the text online, and from there, you can locate the passage about Valak.
The Sixty-second Spirit is Volac, or Valak, or Valu. He is a President Mighty and Great, and appeareth like a Child with Angel's Wings, riding on a Two-headed Dragon. His Office is to give True Answers of Hidden Treasures, and to tell where Serpents may be seen. The which he will bring unto the Exorciser without any Force or Strength being by him employed. He governeth 38 Legions of Spirits, and his Seal is thus.
Now that we've identified the context of the Warrens and the origin of Valak, we arrive at The Nun . . . which, as I said, isn't really based in reality at all.