Katherine Rundell considers herself to be someone who works “quite slowly”. Lord help the rest of us, then. At 36 she has written five adventure novels for children, plus an award-winning play and three books for adults, including a delightful compendium of animals, The Golden Mole, and her biography of John Donne, Super-Infinite, which she described as “an act of evangelism” for his poetry.

She was the youngest person to win the Baillie Gifford prize for nonfiction and the youngest female fellow at All Souls, Oxford’s most elevated college. In case you imagine her to be some cloistered academic, she has scaled Battersea Power Station, mastered the flying trapeze, flown a small plane and been arrested for protesting outside a nuclear weapons base. She gives every impression of being a woman who does six impossible things before breakfast — indeed, when we meet she confirms that she has been up since 5am working on a “secret” screenplay of a children’s classic.

In person, Rundell has an extraordinary, otherworldly aura, like a Tolkien-esque elfin queen. She speaks in immaculate, lyrical sentences. “I want to build an engine of astonishment in children’s hearts and give them the key to turn it on,” she tells me shortly after we meet in the basement of her publisher, Bloomsbury.

Impossible Creatures, her latest novel, really did take a long time by her ludicrously high standards. The first seed of the idea — a girl with a flying cape, a secret cluster of islands where the creatures of mythology still exist — arrived in 2016. There were aborted starts; over 100 hours spent researching lavellans and manticores and ratatoskrs in the Bodleian library; and other projects that kept getting in the way. But really, she says, it took such a long time because she was trying to convey the “whole scope” of what it is to be human. “If you could see the horror we have done and you could also see the wild generosity that humans are capable of — would you say ‘yes’ or ‘no’?” Which is not the sort of question one imagines troubling many of the authors in the children’s best-seller list.

The answer that Rundell arrives at — in one of the most heart-rending passages I’ve read in a long time — is “yes”. And I don’t think I’m being too hyperbolical when I say that Impossible Creatures, the first of a planned trilogy, is an instant classic, one to place alongside works by her fellow Oxford children’s authors Lewis Carroll, JRR Tolkien, CS Lewis and Philip Pullman. Personally, I think her writing is stronger than JK Rowling’s. It’s the story of a boy, Christopher, who rescues a pet griffin in rural Scotland and accidentally uncovers a portal that leads to a magical archipelago. There he encounters a girl called Mal, who owns a flying coat. But she is in jeopardy — a man is trying to kill her.

She gives every impression of being a woman who does six impossible things before breakfast.

Rundell is undoubtedly one of the most arresting figures to emerge in the books world in recent years. This year at the Sunday Times Young Writer of the Year award, for which she was shortlisted, I witnessed men go to pieces in her company. One well-known writer told me: “I know several grown women have crushes on Katherine Rundell … she makes cleverness appealing like no one before her.”

But it’s not the erudition that makes the story such a delight — it’s the spirit of adventure, the eccentric characters, the exotic creatures and the jokes about snot and filth, as well as the palpable sense of a world in peril. “I’ve always wanted to write books that put children right at the end of their capabilities because I do think that we have consistently wildly underestimated children in our culture,” she says. “I think we have underestimated the capacity that they have for profundity of love and for endurance, and of course for cruelty.”

Rundell thinks about darkness a lot — “all the time”. And when I mention that her stories are surprisingly violent in places, she replies: “I think children’s books need to acknowledge darkness because children — especially now with the way the media works — they have such access to information, they are aware of darkness in the world … if you acknowledge to them that there is darkness, then they may believe you when you insist on light.”

Rundell learned at a young age that “there is always going to be joy and sorrow very, very close up against each other”. When she was nine, her elder foster sister became ill and later died. She believes it’s no coincidence that she writes for children that age. At the time her family lived in Zimbabwe, where her father was a diplomat (he now works for the UN in Mali), and she and her siblings (an elder biological brother and two elder foster sisters) had a wild, carefree existence, riding horses among zebras and building rafts across crocodile-infested waters. “I was a very generously minimally parented child,” she says with a wry smile. “My parents allowed us a huge amount of freedom, so we did things that would have registered as profoundly mischievous if anyone knew, but they never did.”

Her mother, a French lecturer, and father were “big night-time storytellers”, she says. “They would tell us imagined stories about all the very obvious creatures — centaurs, unicorns, dragons — and as a child I found it such a source of straightforward delight.”

“I know several grown women have crushes on Katherine Rundell.”

With her new series she wants her reader to “really feel what it would actually be like to meet those creatures … How would you know the brush of their wings?” But the world she has created is, like our own, under threat. The “glimourie” — the magic that has sustained the mythical world — is fading, meaning “an entire ecosystem is ruptured”.

It became clear that she was an ardent environmentalist on the night of her Baillie Gifford win. After passionately kissing her partner, the film agent Charles Collier, she got up on stage to announce that she was donating half the $62,000 prize money to an ocean-based conservation charity and the other half to supporting refugees. While acknowledging that the prize has changed her life, she expressed misgivings about Baillie Gifford’s investments in fossil fuels, which this summer prompted a writers’ boycott of the Edinburgh Book Festival, also sponsored by the firm.

Rundell still regularly goes on climate change demonstrations — although she hasn’t been arrested since she was put in a police cell aged 19 for protesting outside Faslane naval base. She sits on a climate committee that has been trying to push Oxford University to divest from fossil fuels. She confesses to being “deeply imperfect in my behavior in ways that will fuel climate change”, but is angry that imperfect behavior is used as leverage to silence people. “People feel if they eat meat or fly, they can’t speak, but we need to look less at individual impeccability and much more into what can be done in a group cause.”

Her stories reflect this belief in the power of collective action, in people coming together to help one another. The young people in her books are often orphans but “they build groups and gangs and tiny enterprises of delight for themselves”, she says. They also encounter kindly grown-ups who aren’t their parents. Rundell, who is a stepmother to Collier’s 11-year-old daughter, wants to “offer children a vision of society where there will be adults who will swing for you.” If Impossible Creatures has a message, it’s that by being generous to others “you might find the finest version of being alive”.

When Mal meets Christopher she says, “I need your help,” which Rundell believes are some of the most “galvanic” words you can say. “We often have an attitude to the vulnerable as if they are morally degenerate — not just a burden but an error. But it is fundamentally the human condition to be vulnerable.”

Katherine Rundell’s Impossible Creatures is out now from Bloomsbury Publishing

Johanna Thomas-Corr is the literary editor at The Sunday Times