The boy protagonist and his grandmother (herself a retired witch-hunter who once searched the world for The Grand High Witch, but could never find her) encounter her by accident in a hotel in England, setting in motion the main plot of the novel.
The grandmother had once been one of the very best of the witch-hunters and has travelled all over the globe trying to track down the fabled and elusive Grand High Witch, who might be the key to finally defeat her evil kind.
The original version of character is relatively asexual compared to the highly sexualized portrayal in the film adaptation, as she is described only in terms such as being young and "very pretty".
She has already tortured and killed thousands of children around the world, and in the book she also keeps three of them just recently turned into frogs in her room, intending for them to be eaten alive by seagulls.
Now she plans to give the witches of England her newest invention, the "Formula 86 Delayed Action Mouse-Maker" to distribute and turn all children in the country into mice so they can be exterminated by their own parents.
One witch with whom she is shown to be actually friendly with is "the Woman in Black", who attempted to kill Luke earlier in the story and is given a larger role in the film.
In the original book's chapter The Triumph, The Grand High Witch's own magic is used against her and her followers during their public banquet at the hotel's ballroom, when they are all transformed into brown mice by a massive overdose of Formula 86.
In the film, they begin turning into mice with black-and-white fur, except for the fuming Grand High Witch, who is last to be affected as she resists the effects due to her sheer power.
A panic and utter chaos ensues, and the hotel staff members begin to violently take on what they think is a sudden infestation, unknowingly massacring the helpless mice-witches as they run around squeaking.
"Her Grandness" herself, in her rodent form and now left without any sort of power whatsoever except still being able to talk even after the overdose, is spotted by Luke, who tells his grandmother not to "let her get away".
In the 2020 film, the role of The Grand High Witch was expanded where she was responsible for turning the grandmother's friend Alice Blue into a chicken by giving her chocolate with a magic potion that did the job.
"[10] The character's monstrous version was prepared by Jim Henson's Creature Shop: "The prosthetics for Miss Ernst's transformation to The Grand High Witch were extensive.
Warner Bros professed they were "deeply saddened"[21] at the news that Robert Zemeckis' film had upset people with disabilities, "it was never the intention for viewers to feel that the fantastical, non-human creatures were meant to represent them.