The Grand High Witch is the supreme leader of all Witches, and the main antagonist of the bestselling novel The Witches. She is the archenemy of the unnamed protagonist in the novel.
Little is known about the Grand High Witch's past, but it is assumable that she had once encountered Luke's grandmother. She attends the annual meetings of all Witches in each country every year to discuss their movements against children. As a rule, she murders one Witch in every meeting to set an example to the others, and also as a display of her ruthlessness and complete lack of empathy. All other Witches, who are feared and renowned for their immense degree of cunning and power, are terrified of her.
In the course of the novel, she poses as the aristocratic Eva Ernst, and gathers in a hotel under the pseudonym of the chairwoman of the Royal Society of the Prevention of Cruelty to Children. During the meeting she declares her enterprise to dispose of all children by feeding them enchanted sweets that will transform them all into mice, and that when they go to school and transform into mice the teachers will exterminate them, not knowing that they are in fact children.
However, when one Witch naively mutters that they couldn't possibly wipe all children out, the Grand High Witch shoots purple laser beams from her eyes at her, incinerating her completely. She then brings in a boy, named Bruno, and tricks him into being the lab rat, quite literally, of her enchanted candy. After he transforms, one of the Witches, Mildred, detects Luke, who has been spying on the meeting, and they transform him as well.
The Grand High Witch menaces Luke further, and soon enough it leads to Luke using their own magic against them. The Witches' meals are spiked with their own enchanted potions that transform humans into mice. The hotel staff, and a great many of the guests, go on a rampage after the transformed mice and begin to massacre them with knives, rolling pins and anything they could find. The Grand High Witch takes longer, possibly because of her sheer power, to transform. She vows to revenge herself on Luke's Grandmother, but the old woman responds defiantly, declaring 'It's your turn!'.
The Grand High Witch is later decapitated by the hotel manager, ending her reign of terror forever.
The Grand High Witch was highly intelligent, arrogant, selfish, strong, enigmatic, malicious, cynical, manipulative, Machiavellian, cunning, astute, ruthless and powerful. She was elitist in controlling other Witches, being callous enough to murder one of her own in every annual meeting to set an example. She was a formidable intellectual who showed strong signs of becoming a psychopath, showing traits such as violence, failure-intolerance, egomania, high intelligence, social manipulation, grandiosity and emotional weakness. She was sadistic in murdering and experimenting on children.
Though little of her powers are revealed, other than fire-control and strong experimental skills, she is obviously an extremely powerful woman.