Ellen Leigh

Ellen Taper Leigh, unbeknown to her family, was the evil leader of the cult of Paimon, revered as "Queen Leigh".

The actress who plays Ellen is uncredited in the film, and her name was not disclosed initially. Ari Aster referred to her as "the sweetest person in the world". She was later identified as Kathleen Chalfant.

Background
Little is known about Ellen, at least to her daughter Annie. Annie describes Ellen as secretive, manipulative and stubborn, that she never admitted to being wrong nor felt guilty about anything. But at times, she could appear to be loving and sweet. Ellen had a private part of her life kept away from the Grahams, her daughter's family.

Ellen was married to Martin Leigh. The couple had a son and a daughter, Charles and Annie. Martin died of starvation when Annie was a baby. Annie only knows that her father's starvation and death were the result of psychotic depression, and her brother Charles had schizophrenia. Charles hanged himself in Ellen's bedroom at 16. He left behind a suicide note accusing Ellen of "putting people inside him".

In reality, Ellen was the leader of the cult of Paimon. She seemed to have married Paimon in a cult ceremony late in life. She was revered as "Queen Leigh" in the cult. Therefore, her son's suicide might have been the result of Ellen's failed attempt to use him as a sacrifice and vessel for Paimon. Ellen had known her friend Joan ever since Joan was a young woman. Together they served Paimon in the cult and worked to bring him into the world.

While Annie might love her mother, she resented Ellen's manipulativeness, which resulted in her husband Steve Graham forbidding her to see Ellen. Annie cut Ellen off when her first child Peter was born. Annie felt guilty afterwards and let Ellen back into her life when she was pregnant with her daughter Charlie. Ellen immediately got herself involved with raising Charlie, and might have even breastfed the girl herself, as shown by one of Annie's miniature works. Unbeknownst to the Grahams, she was grooming Charlie to become Paimon's temporary vessel. Ellen's involvement seemingly resulted in Charlie grown up to be a morbid teenager.

Ellen had dissociative identity disorder (DID), which got extreme at the end of her life, and dementia as well. She and Annie were not on speaking terms until she got sick and had to move in with Annie's family before hospice. She died at the age of 78. Joan took over the cult and began to implement the next step in summoning Paimon into the world.

Hereditary
Many members in the cult of Paimon pay respect to Ellen in her funeral. Annie is quite surprised by the appearance of these people that she has never met before. Annie gives a eulogy about the strained relationship she had with her mother. Charlie finds that someone among the strangers is paying herself special attention. Then she sees a woman rubs something on Ellen's lips.

That night in her workshop, Annie finds a letter in Ellen's stuff, asking Annie not to despair her losses as they will be worth it in the end. Then she briefly sees Ellen's apparition. Annie tries to find out the meaning of the apparition on the internet. She even makes a miniature room model where a figure of Ellen, likely representing her apparition, is standing at the doorway, looking at Annie and Steve's figures asleep on the bed.

Joan and the cult plot against the Graham family after Ellen's death. They steal Ellen's body from her grave one week after her funeral. Then they orchestrate several supernatural events to befallen on the Grahams. One day, Charlie sees Ellen's apparition sitting in a patch of grassland, surrounded by flames. That night she gets killed in a traffic accident devised by the Paimon cult.

Joan has planted herself in a grief recovery group that Annie has previously attended. She tricks Annie into performing a demonic ritual which apparently binds her whole family to Paimon. As her son Peter gradually becomes more hysteric, Annie figures out Joan has known Ellen all along. She goes through Ellen's stuff again and finds out that Ellen and Joan were in league with Paimon. Then she discovers Ellen's beheaded corpse has been placed in the attic of her house, with Paimon's symbol drawn above it in blood.

Despite Ellen's death, her plans pull off posthumously as her daughter's entire family fall victims to the cult of Paimon. Annie fails to undo the mistakes she has made, and gets Steve and herself killed. Eventually Peter, in a prolonged weakened mental state, jumps out of a window in the attic, thereby allowing Paimon to occupy his body, completing the summoning ritual.

Trivia

 * It is possible that Paimon summoned Ellen's spirit to haunt the Graham family in order to carry out his plan, as it is said in The Lesser Key of Solomon that Paimon is adept at making spirits appear.