In the new horror movie The Nun, a demonic nun called Valak haunts the dark halls of an monastery in Romania. The movie follows a young nun-in-training named Sister Irene (Taissa Farmiga) and a priest named Father Burke (Demián Bichir) who go to the Carta Monastery to investigate a nun's mysterious suicide. As you can imagine, it's not quite as straightforward as other suspicious deaths, because, you know, a demonic nun roams the halls of the monastery. To make things more interesting — especially for those who get easily spooked IRL, Valak in The Nun is actually based on a long-recorded history of mythology that suggests the demon could be real.
In The Conjuring 2, the movie to which The Nun serves as a prequel, paranormal investigator Lorraine Warren (Vera Farmiga) has a vision in which a ghostly nun reveals that its name is Valak. That same nun haunts Sister Irene and Father Burke in The Nun, but that name isn't made up by The Nun's creators. While different iterations of "Valak" exist in various mythologies — the demon is also referred to as Volac, or Valak, or Valu, according to Grimoire Encyclopedia — the demon from The Nun most likely gets its name from the well-documented demonic entity.
Not only is Valak a real-life mythologized demon, but it's one of the most powerful demons in history. One of the most essential books about demons is called The Lesser Key of Solomon, which originally came out in the mid-17th century, though according to Grimoire Encyclopedia the so-called "demonology" some of the book's sources date back to the 13th century, if not earlier. The Lesser Key of Solomon states that Valak is a demon that governs over 30 legions of evil spirits, and it appears as a child with angel wings that rides on a two-headed dragon. That doesn't sound anything like the terrifying nun which the demon appears as in The Nun, but it seems like the level of malevolence remains the same throughout the movie and lore.
As the name describes, The Lesser Key of Solomon is partially comprised of visions which King Solomon received during his communications with God, per Grimoire Encyclopedia. Of all the demons listed in the foundational book, Valak receives the name, the Grand President of Hell, per Digital Spy. Even if the appearance of Valak in The Nun differs from popular mythology's descriptions of the demon, The Conjuring 2 suggests that the demonic nun could also earn a title such as the leader of the underworld.
As mentioned before, The Conjuring 2 reveals that Valak was powerful enough to prevent Lorraine Warren's clairvoyant powers from working, so that Valak could possess her. In the end, Lorraine remembers that Valak revealed its name and she condemns the demon back to Hell by calling it by its name.
The changes that The Nun makes to the long-recorded history of Valak likely don't end with the difference in appearances, but it makes the new horror movie so much creepier just knowing that a demon named Valak actually could exist. For horror-lovers, that just about makes The Nun as good as a scary movie can get.