JK Rowling is set to publish a special Harry Potter story on her website as a Halloween surprise for fans.

The original tale will focus on cruel Hogwarts professor Dolores Umbridge, the witch who first appeared in Harry Potter and the Order of Phoenix as the Minister for Magic's Senior Undersecretary, played by Imelda Staunton in the films.

Umbridge memorably ordered young wizard Harry to use a Blood Quill and scar himself with the phrase, "I must not tell lies".

"Umbridge is not only one of the most malicious Potter characters, she is the only person other than Lord Voldemort to leave a permanent physical scar," reads a statement on Pottermore.com.

The website dropped a few hints for fans, promising a "rich, 1700-word back story about Umbridge's life, filled with many new details, as well as Rowling's revealing first-person thoughts and reflections about the character".

The story is expected to appear online on Friday 31 October.

Rowling released another Harry Potter story online last July, which saw a 33-year-old, greying Harry reunite with Ron Weasley, Hermione Grainger, Luna Lovegood and Neville Longbottom for the final of the Quidditch World Cup. It attracted so much traffic that Pottermore temporarily crashed.

Harry Potter stars then and now Show all 8 1 /8 Harry Potter stars then and now Harry Potter stars then and now Daniel Radcliffe The transformation of Daniel Radcliffe from the small boy pictured (right) at the first Harry Potter premiere has grown up into a critically acclaimed actor, seen earlier this year walking the red carpet. Harry Potter stars then and now Emma Watson Emma Watson, seen (right) at the very first Harry Potter premiere, has gone from acting strength to strength, despite declaring at one point she would be quitting acting to pursue other projects. Harry Potter stars then and now Rupert Grint Rupert Grint, who plays Ron Weasley in the films, pictured at the very first premiere (aged 13) and earlier this year. Harry Potter stars then and now Matthew Lewis Possibly the biggest transformation is Matthew Lewis, who played Neville Longbottom in the films. Pictured (right) at the premiere of the Goblet of Fire and (left) earlier this year. Harry Potter stars then and now Tom Felton Harry's on-screen tormentor Draco Malfoy, played by Tom Felton, who commented that after finishing Harry Potter he was looking forward to playing a less spiteful character. Harry Potter stars then and now Robert Pattinson Robert Pattinson only appeared in one of the Harry Potter films, despite this it helped to launch him as a globally recognised actor today. Pictured (left) at the premiere of Goblet of Fire and (right) earlier this year at an Australian ceremony Harry Potter stars then and now Bonnie Wright Harry's on-screen long-term love interest Ginny Weasley was played by Bonnie Wright, pictured (right) at the Prisoners of Azkaban premiere and earlier this year. Harry Potter stars then and now James and Oliver Phelps James and Oliver Phelps played the wise-cracking Weasley twins and the two were picture together again earlier this year.

Earlier this week, Rowling was forced to debunk rumours that she has recently finished a romantic novel.

Newspaper reports suggested she had been "celebrating" the book's completion in a London bar, but the allegations proved false.

"I sometimes have a drink even when I haven't finished a book," she wrote on Twitter. "Yes, that's how rock and roll I really am."