At the end of the movie Hereditary (2018), leviating, Annie beheads herself with a piano wire in front of Peter.
Why did she behead herself?
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Annie is possessed by Paimon. For this reason, She chops her own head off. The decapitations were needed for the ritual.
Charlie sticks her head out of the window in an attempt to ease her breathing, and is decapitated by a wooden post. The post bears the same symbol that is on the necklace Annie's mother gave her (and continues to crop up throughout the film). An internet search identifies it as the 'Seal of Paimon' from the real-life 'spell book' The Lesser Key of Solomon.
Ari Aster: The cult of "Paimonists" dug her up and put her there (as per grandma's instructions).
In the movie, Peter is told that the trinity has been destroyed and that Paimon now reigns.
Ari Aster: Well, Joan does mention that the cult has "rejected the Holy Trinity" at the end, so that's there.
The decapitated bodies, who have been positioned into a worshipful position (They're all arranged in a triangle), represent an "Unholy Trinity": the Maiden, the Mother, and the Crone (the Triple Goddess of certain forms of Neopaganism)
In the illustration of the book, Paimon has three heads.