Terry Pratchett, whose more than seventy award-winning fantasy novels captured the imaginations of readers worldwide, died on Thursday at his home near Salisbury, England. He was sixty-six.

According to his publishers, Pratchett died of a rare form of early-onset dementia.

The news came in a series of tweets in the author’s Twitter feed, in the voice of one of his literary characters, Death: “AT LAST, SIR TERRY, WE MUST WALK TOGETHER.” The following tweet read, “Terry took Death’s arm and followed him through the doors and on to the black desert under the endless night.”

He had completed this last book, Raising Steam, another in the “Discworld” series, last summer.

“Discworld,” a primary setting of Pratchett’s books, is a disc-like planet balanced on the back of four elephants who stand upon the shell of a giant turtle. First introduced in The Colour of Magic (1983), the planet grew in inhabitants, coming to accommodate witches, trolls, and other creatures, who, as New York Times writer Bruce Weber put it in his obituary, “often seem to re-enact the follies of Englishmen and other Earth people.”

“Mr. Pratchett often wrote with eyebrow arched and tongue planed firmly in cheek; in the behavior of his mythical creatures it was hard to miss the barbs being tossed in the direction of humanity,” Weber wrote.

Pratchett was born on April 28, 1948 in Beaconsfield, Buckinghamshire. After dropping out of school, Pratchett worked as a journalist at a newspaper. He published his first novel, The Carpet People, about a tribe of people that lived on a vast carpet, when he was in his early twenties.

Some of his other books include The Unadulterated Cat, Good Omens (which he co-wrote with Neil Gaiman), The Dark Side of the Sun, and Dodger.

Before his death, Pratchett campaigned for dementia awareness. He contributed a million dollars to Alzheimer’s research and was an advocate for the legalization of assisted suicide.

John Scalzi, an award-winning science fiction author, said that Pratchett’s incorporation of humor into his writing revolutionized the genre.

“The thing that most people don’t realize is that it’s not easy to be funny, and it’s not easy to be funny in science fiction and fantasy,” Scalzi said. “A lot of humor is rooted in relatability and when you’re dealing with fantasy and you’re dealing with science fiction you’re dealing with elements outside of the everyday.”