The most frightening villains are the ones who know us. They prey on our particular vulnerabilities and threaten the things we love. They have the power to activate a personal nightmare.

Rarely has this brand of malice been exercised to more horrific effect than by Dolores Umbridge, Professor of Defence against the Dark Arts at Hogwarts school and sometime employee of the Ministry of Magic, who makes her first appearance in the fifth book of the Harry Potter series and retains her sickening hold until the latter part of The Deathly Hallows.

Woe betide any reader who assumes this villain is one “for children”. JK Rowling’s creation enacts a sophisticated, careful sadism, both within and beyond Hogwarts. She relishes each moment of the suffering she inflicts.

It is in keeping with the control she exerts that Umbridge is first glimpsed in shadow, when Harry is summoned before the “Wizengamot” - the Ministry of Magic’s high court - charged with the illegal practice of spells, in the opening chapters of The Order of the Phoenix.

In this setting, heavy with the air of inquisition, he is dimly aware of an apparently unoccupied space to the right of the Minister for Magic, Cornelius Fudge. The shape of a person is suggested, although she remains just out of view. It is only towards the end of the hearing that “a pale toadlike witch” emerges. In her capacity as Undersecretary to the Minister, Dolores is immediately a presence, sinister and officious. It is a mere whisper of the trials she will bring, but her “silvery laugh”, incongruous in so frog-like a woman, makes the hairs stand up on the back of Harry’s neck.

In this book, the gathering of dark forces removes any remaining sense of school as a safe haven. When Harry spreads the rumour that the Dark Lord (Voldemort) has returned, Umbridge - cleaving to ministry regulations - seizes her moment to punish him for his “lie”. We already understand her hatred of him but her new power to enact it is signalled via a chilling instruction: “Detention… Tomorrow evening. Five o’clock. My office”.

The walls of Umbridge’s office are covered with a “collection of ornamental plates, each decorated with a large technicolored kitten wearing a different bow around its neck”. The twee ornamentation is not, in isolation, the stuff of nightmare but, in conjunction with the deep-set evil of its occupant, it becomes a stomach-churning portent of punishments to come.

Instructed by Dolores to write out the lines “I must not tell lies”, Harry is handed a “rather special” quill of her own creation. It has, we discover, “an unusually sharp point”. As he bends to the parchment, he is assured by the professor, with the suggestion of a smile, “oh, you won’t need ink”.

Scratching out the first words, Harry feels an agonising pain on the back of his right hand, and the terrifying realisation strikes him that each scratch of the quill on paper will carve into his own skin, the words “appearing in shining red ink...... as though traced by a scalpel”. To extend the torture, the wound heals itself on the completion of each line, leaving the skin red and inflamed but able to receive the next cut.

There is a terrible intimacy to this encounter, in which teacher and pupil seem to enter into an appalling collusion. “You know deep down ... you deserve to be punished, don’t you Mr Potter?” Umbridge intones. In turn, Harry refuses to give his tormentor the satisfaction of seeing any show of pain or weakness, continuing to etch lines in his own skin in the corner of her room.

To read this scene for the first time is the literary equivalent of peeping through one’s fingers at the cinema screen. We eke it out in squeamish pleasure yet cannot wait for its end. Perhaps most shocking of all is that Harry feels initially unable to speak of the torture to his closest friends, Ron and Hermione. It is as if there are some horrors that cannot be told. To share the true nature of what recurs in Umbridge’s office would be to make it real. Here is a villain capable of taking away the last vestige of how things should be.

Through the remaining two books, Umbridge is fired by her fierce hatred for anyone not of “pure blood” (magical heritage), engineering the forcible removal of groundskeeper Hagrid from the school. As her fanaticism pushes at its limits, so does her love of childish things. Girlish and increasingly high-pitched in voice, she “trills” her way through the torture she inflicts on her innocent victims.

In a recent essay, published, fittingly, on Halloween, Rowling fills out the relatively scant background details about Umbridge’s early life. They do not humanise her. Despite her obsession with the superiority of “pure-bloods”, her own ancestry proves a falsehood. She is, we learn, the daughter of a Muggle mother.

In branding Harry’s skin, Umbridge is the only one to mark him, other than Voldemort himself, who is responsible for the scar on his forehead. As an incarnation of evil, her mark is equally fixed on the imagination of her readers. We are never entirely free of her - even when she is safely locked up in Azkaban.

•This article has amended to clarify that Dolores Umbridge is Professor of Defence against the Dark Arts, and the wound inflicted on Harry appeared on his hand.