As George RR Martin takes his time over a new instalment from the Game of Thrones world, much to the frustration of his legions of fans, fellow fantasy writer Robin Hobb has finished the final book in her 16-part series. More than 20 years ago, Hobb set out to tell the story of the bastard son of a prince, assassin FitzChivalry Farseer. Sixteen brick-like volumes and thousands of pages later, wars have been fought, magic wielded, dragons resurrected, fair maidens lost and won and lost again, and she has pulled her epic tale to a breathtaking conclusion in Assassin’s Fate, published this spring.

Hobb, who is softly spoken and carefully deliberate in her choice of words, sounds relieved about coming to the end of a story she felt she had finished at least twice before. “It feels like I worked on it for ever,” she says, jet lagged in a Kensington hotel after flying in from the US, where she lives on a small farm in Washington state with her husband and two dogs.

Embarking on the final novel was, she says, a scary prospect. “What was hardest for this one was [having] 15 volumes of backstory – trying to keep it all coherent and cohesive so I’m not contradicting something I said before.”

And she was wary of the trap many fantasy writers fall into of letting their story get away from them, unravelling hopelessly over an endless series of volumes. “It terrifies me as much as anybody. It’s really hard to say when you’re this close, ‘Did I get that right or is it just diffusing?’”

Hobb is the author whose books George RR Martin described as 'like diamonds in a sea of zircons'

Hobb is the author whose books Martin described as “like diamonds in a sea of zircons”, the writer to press on those who turn up their noses at fantasy. Her writing is superb, her world-building irresistible, her storylines both hell-for-leather exciting and deeply introspective. And the way she pulls together multiple disparate strands from the preceding volumes in Assassin’s Fate is a joy to read.

Good fantasy, Hobb believes, is about “lowering the threshold of disbelief so the reader can step right into the book and not feel blocked out by something that’s impossible or at first glance silly. And I think silly is more dangerous than impossible.”

It is also, as Martin knows so well, about not being afraid to draw the final curtain for your characters when the time comes. “Nobody gets to go on for ever. If you put a little magical umbrella over your characters and say ‘yes, we’re going to scare you a little bit but ultimately you know that at the end of the book everything is going to be much the same way it was when we started the story’, well then, why write the story, what’s the point?”

Peter Jackson’s film adaptation of The Return of the King … ‘The filming of The Lord of the Rings made people realise these stories can be taken seriously’. Photograph: Alamy

Hobb had been dabbling in writing for years before she really committed. “It would come and go,” she says. “I had been trying to write before I had kids, and then I had kids and I kept on writing, working mostly on short stories, mostly for children.”

She sold these stories to magazines, but then decided to try writing in the genre she preferred reading: science fiction and fantasy. She published her first novel, Harpy’s Flight, at the age of 30, in 1983, under the name Megan Lindholm (her real name is Margaret Ogden). The story of a woman whose family has been slaughtered, and who is fleeing from vengeful harpies, it “got good reviews and mediocre sales”, says Hobb. “It did OK but it never earned out, and so in the US I was a midlist writer for a lot of years. Midlist means it’s a nice little addendum to your income but don’t quit your day job. And I had a number of those. When I was writing Harpy’s Flight I was working as a waitress. And I had the kids and then ended up with a tiny farm, so I was trying to keep that going. And my husband was away at sea.” (He’s a submarine engineer, whom she married at 18.)

She was not put off. “There was no discouraging me. Writing is partially an obsession or a compulsion. People who are meant to do it are going to do it whether they are published or not.”

Further titles under the name Lindholm followed, before the story that would become Assassin’s Apprentice, the first she published as Robin Hobb, began to flow. The book is told in the voice of FitzChivalry Farseer, or Fitz, an illegitimate son who is dumped at the royal court in the medievalesque kingdom of the Six Duchies. His father is dead, and he is raised in the stables, rejected by all his family apart from his uncle Chade, who trains him as an assassin. It turns out Fitz also has a talent for magic, which in this world is a dangerous, rare ability known as “the Skill”.

“I quite clearly remember, because I was working on something else – and that’s always where you get your best ideas – thinking, ‘What if magic was an addiction, and what if that addiction was destructive, so you really want to continue working magic, even though you know that it’s destroying your mind or your health?’ And that’s the Skill,” says Hobb. “But I didn’t have much else to go with it at the time, so I wrote it on a torn piece off an envelope, and I put it in my desk drawer. It was there for years, and other bits and pieces of ideas accrued around it.”

The voice she was writing in was, she found, very different from Megan Lindholm, who is “a little more snarky, a little more sarcastic, a little less optimistic, less emotional. Fitz is cagey, he doesn’t always admit the full truth about anything, but he really says what he’s feeling.” So she and her publishers came up with Robin Hobb as a pen name for this new voice. It was short, fitting on the cover nicely; it was androgynous, and “if you say Robin the first thing people usually think of is Robin Hood or Robin Goodfellow, so it’s got all these nice little nuances tied into it”.

George RR Martin has helped to address ‘a lot of misapprehension’ about fantasy.

Photograph: Murdo Macleod/The Guardian

Hobb drew the geography of the Six Duchies from Alaska and the Pacific Northwest, where she and her husband lived for many years. She wove in the concept of “the Wit”, a vilified “beast magic”. And a character who had been a walk-on part, the mysterious, enigmatically gendered Fool, sidled on to centre stage, helping to turn Assassin’s Apprentice into something different from the usual swords and sorcery fantasy.

“When I first began writing the book, the Fool had exactly one sentence in the outline. But as he came more and more into the story, whether I wanted him there or not, it affected the plot and got woven through it until he was one of the major characters. He became a driving force that complicated the plot, had his own agenda,” she says. “He’s the wild card.”

Assassin’s Apprentice was the first book in a planned trilogy. It turned out to be a gamechanger for Hobb, who has now sold more than 1.25m copies of her books in the UK alone. She finished the trilogy, not particularly planning to return to the story of Fitz and the Fool, and wrote another set of books in a different corner of the same world, featuring living ships.

And then she found herself with another story to tell about Fitz. The Tawny Man trilogy was published between 2001 and 2003, followed by the Soldier Son trilogy, set in an unrelated fantasy realm, and then four books set in a far corner of Fitz’s world and featuring a new cast of characters, the Rain Wild Chronicles. In 2014 she returned to Fitz and the Fool for a final trilogy. There are sentences in Assassin’s Fate, she says, that she’s had in her head for more than 20 years.

“At the start I was very sure it was just a trilogy and that was done with it. And then I was going to write the Liveships [The Liveship Traders trilogy] which was set in the same world but with different characters. I kind of knew there would be interplay in some distant future between these characters and the events that they’d triggered, but I never thought I’d actually write all the way there,” says Hobb.

“It’s like watching a piece of clockwork, the old clocks where the shepherd and the sheep come out and strike a chime – that’s kind of like what happens when you’re writing in a really big world. You set off events, and then you see coming into view what they are triggering.”

Hobb believes that fantasy in general is in a good place right now. “I think the lid’s been taken off in some ways. When I first started I was writing paperbacks, and I was told, don’t write anything more than 250 pages long, 275 tops, because a paperback binding isn’t going to hold anything more than that. So you’ve got a really strict limit there.”

Then the late Robert Jordan came along, with his Wheel of Time fantasy series. “It looked like a brick. I had never seen a book shaped like that before and I thought, ‘They did figure out how to bind that! OK, maybe I’m not going to cut this because it won’t fit in a paperback binding.’”

Hobb is a friend of Martin, whose writing she first spotted at a convention decades ago. She feels he, along with the Lord of the Rings films, has helped to address “a lot of misapprehension” about fantasy – the feeling that “‘Oh, it’s for children, it’s about elves and fairies and pixies and unicorns’ … I don’t think people realised what it actually was,” she says. “And certainly the filming of The Lord of the Rings made people stop and think that these are stories that can be taken seriously … It became something that you didn’t have to be embarrassed about carrying.”

While she finally succumbed to watching Game of Thrones – she prefers the books – there are no plans in the works for film or television adaptations of her own novels. “It would have to be a situation of somebody I trusted and felt would respect the characters. Not the author so much, but the characters.” For now, Hobb is relishing having completed such an epic work, after living with her characters for 25 years. “It’s still a little bit hard to believe,” she says. “As always, you know what happens next in that world, the things that go on, but this big piece of it is finally over. The world goes on – but everybody has an ending.”