Anthony Lane captures what actually makes Hereditary by Ari Aster such a terrifying movie:

Should you want to measure the psychological disturbance at work here, try comparing “Hereditary” with “A Quiet Place.” That recent hit, for all its masterly shocks, is at bottom a reassuring film, introducing people who are beset by an external menace but more or less able to pull through because, as a team, they’re roped together with enough love to fight back. “Hereditary” is more perplexing. It has the nerve to suggest that the social unit is, by definition, self-menacing, and that the home is no longer a sanctuary but a crumbling fortress, under siege from within. That is why there are no doctors in Aster’s film, and no detectives, either, urgently though both are required; nor does a man of God arrive, as he does in “The Exorcist” (1973), to lay the anguish to rest. Nothing, in short, can help Annie, Steve, and the kids, and they sure can’t help themselves, stationed as they are inside their delicate doll’s house of a world. There is no family curse in this remarkable movie. The family is the curse. Klokk.

While watching, I noticed how the everyday items of family life are turned into instruments of suffering: chocolate cake, a car, a book, a piano, a tree house. I was reminded of Elaine Scarry’s book The Body in Pain, where she talks about the use of everyday items in torture:

The room, both in its structure and its content, is converted into a weapon, deconverted, undone. Made to participate in the annihilation of the prisoners, made to demonstrate that everything is a weapon, the objects themselves, and with them the fact of civilization, are annihilated: there is no wall, no window, no door, no bathtub, no refrigerator, no chair, no bed.

The immediate physical threats are not the true source of horror within the universe of Hereditary. It’s the total destruction of any safe or normal place that could provide comfort against what’s happened.