This article is going to go into details about the ending, so BEWARE THE SPOILERS!
Hereditary is a devastatingly effective film. It's a horror film, a tragedy, and a dark psychological drama, depending on which part of the movie speaks most to you. It's a very intense experience that will remain the most talked about horror film in 2018.
The movie is ultimately about the price an innocent - if frazzled - family pays for their late grandmother's efforts to bring the demon Paimon back to earth by offering the lives and souls of her descendants. There are elements of The Babadook in the degenerating insanity of the mother and there is clear callbacks to Rosemary's Baby with the sinister conspiracy drawing in on the family. As the movie takes places within the claustrophobic confines of their dollhouse home, the occult elements become stronger until it ends in fire and blood and decapitations and demonic possession.
Some questions have arisen at the ending, especially around the mythology of Paimon, the demonic force that takes over the last remaining member of the Graham family. After getting over the shock of seeing the film and spending way too much time reading up on the mythology of the Paimon cult, I've found some interesting tidbits I'd like to share with you all.
The short answer is that Paimon is a demon from hell, a trickster that kneels only to Lucifer himself, and is believed to manifest wearing a crown and being accompanied by horns, something that the score plays up in the final scene of the film.
For more on the history of Paimon and his occult origins, Lifehacker dug this up.
At several points during the movie, an entity known as Paimon is mentioned. This is a spirit or demon that appears in a number of Medieval grimoires (AKA spell books), including the Lesser Key of Solomon, Pseudomonarchia Daemonum, Book of the Office of Spirits and Dictionnaire Infernal. If you were a freaky wannabe necromancer in the 1600s, these books would have definitely been on your pull list.
According to grimoire lore, Paimon is an agent of Lucifer, described variously as a duke of the underworld, a fallen angel and king in the West. He was said to rule 200 legions of spirits and had the power to grant otherworldly riches to anyone who summoned him.
To perform a summoning, the occultist was required to first draw the Seal of Paimon:
The other half of the story is, of course, the way that Peter becomes the host of Paimon. Much of the tragedy that he suffers is due directly to the cult. When he and Charlie are first attending the party, we can see the symbol of Paimon carved into the post that will eventually take Charlie's life. We know that Charlie and Peter's deceased grandmother has tried to use her family as hosts before; during group therapy Annie Graham talks about having a brother who committed suicide, claiming that his mother was trying to force people into his body. In addition, Charlie says that her grandmother wishes that she had been born a boy, as Paimon can only possess a male vessel.
Though the movie starts with the grandmother's passing and we never meet her in any way over the course of the story (beyond her corpse trespassing in the family attic), she casts a massive shadow over the family. She warns her daughter of the doom about to befall her family through hidden notes in books, she sends her acolytes to teach Annie the ritual used to summon Paimon, and we sense that her presence remains in the house, pushing the family to its horrible end. The movie is often a depressing look at toxic families were abuse is a multi-generation issue. As the grandmother destroys Annie's life, Annie's ambivalence towards motherhood has turned Peter into a nervous wreck.
This isn't a fun or pretty movie. It's scary and sad, and there's a sense of spiraling into inevitable doom. But it's also one of the best horror movies I've ever seen.
Now, if you'll excuse me, I'm going to go hug a puppy.