Best-selling au thor Sir Terry Pratchett has died aged 66 after a very public struggle with Al zheimer's disease.
His death was announced to fans on Twitter in a ser ies of messages shortly after 3pm.
They read: "AT LAST, SIR TERRY, WE MUST WALK TOGETHER.
"Terry took Death's arm and followed him through the doors and on to the black desert under the endless night.
His daughter Rhianna later wrote: " Many thanks for all the kind words about my dad. Those last few tweets were sent with shaking hands and tear-filled eyes."
The news was confirmed by his publisher, Larry Finlay, who said he was "deeply saddened" by the loss of one of the world's " brightest, sharpest minds".
Mr Finlay, managing director at Transworld Publishers, said Sir Terry "passed away in his home, with his cat sleeping on his bed, surrounded by his family".
He completed his last book - set like so many of his best-sellers in Discworld - last year.
Mr Finlay said: "In over 70 books, Terry enriched the planet like few before him. As all who read him know, Discworld was his vehicle to satirise this world; he did so brilliantly, with great skill, enormous humour and constant invention.
"Terry faced his Alzheimer's disease (an 'embuggerance', as he called it) publicly and bravely. Over the last few years, it was his writing that sustained him. His legacy will endure for decades to come.
"My sympathies go out to Terry's wife Lyn, their daughter Rhianna, to his close friend Rob Wilkins, and to all closest to him."
The comic universe he created in Discworld - a flat disc balanced on the backs of four elephants standing on the back of a giant turtle - made millions laugh and made them think as well.
His sense of fun made him stand out in the often po-faced world of fantasy literature - he would turn up at conventions wearing a T-shirt saying: " Tolkien's dead, JK Rowling said no, Philip Pullman couldn't make it. Hi. I'm Terry Pratchett."
Towards the end of his life, h e used his fame and wealth to campaign for a greater awareness of dementia and assisted dying.
In 2011, he featured in a documentary about suicide in which he followed a man with motor neurone disease to the Dignitas clinic in Switzerland to see him take a lethal dose of barbiturates.
Asked why he wanted to make the film, he said it was because he was "appalled" at the state of the law.
A year earlier, he had used the prestigious Richard Dimbleby Lecture to call for assisted suicide to be legalised.
Hilary Evans, director of Alzheimer's Research UK, said the death of Sir Terry would have "a profound effect on bo th literature and the 850,000 people who live with dementia".
She said his announcement of his own diagnosis was "a watershed moment" and "a call to arms for society to talk about dementia and take steps towards defeating it".
Prime Minister David Cameron said: " Sad to hear of Sir Terry Pratchett's death, his books fired the imagination of millions and he fearlessly campaigned for dementia awareness."
Sarah Wootton, chief executive of Dignity in Dying, called him "a committed campaigner" for "assisted dying".
She said: "S ir Terry was fond of saying 'It's time we learned to be as good at dying as we are at living' and his brave approach to confronting issues of death, including his own, was a heartfelt demonstration of dignity."
Alzheimer's Society chief executive Jeremy Hughes said Sir Terry had "fundamentally changed the way dementia is seen and understood".
Sir Terry started out as a reporter for the Bucks Free Press, later joining the Western Daily Press and the Bath Chronicle before becoming a press officer for the Central Electricity Generating Board, with responsibility for three nuclear power stations, in 1980.
He published his first novel, The Carpet People, in 1971 but his career really took off after the publication of the first Discworld book, The Colour Of Magic, in 1983.
His books sold millions of copies worldwide and were translated into more than 30 l anguages.
Asked about his success in 2009 when he was knighted, he said: " Most writers don't make much money, they only happen to make some if they're standing in the station when the gravy train comes in.
''I thought I was lucky to make some money, then lucky to make a living, then lucky to be a millionaire.''
A JustGiving page in memory of the writer, who lived n ear Salisbury in Wiltshire, had already raised thousands of pounds for an Alzheimer's charity in his memory less than an hour after his death was announced.
Writer Neil Gaiman, who collaborated with Sir Terry on the novel Good Omens, said: "My apologies to all news outlets, TV stations, everyone asking for statements about Terry's death. I don't think I can say anything today.
"He was my friend for 30 years and a month. I miss him. Donate to Alzheimer's research and make it so things like this don't happen."