When Rebecca Yarros pitched her publisher a sexy fantasy about telepathic dragons and their riders, she thought it might be a tough sell.

She’d built a career and a dedicated following writing romances, often drawing on her experience as a military wife. What she was proposing was wildly off-brand: an epic fantasy series with dragons, griffins, magic and political intrigue.

To Yarros’s surprise, her publisher, Entangled, loved the idea, and wanted to launch a new fantasy imprint with it. Over a feverish few months, Yarros crash-wrote “Fourth Wing,” an intricately plotted 500-plus-page narrative that takes place at an elite war college, where two dragon riders feud, then fall in love. She was stunned when she learned they were printing more than 100,000 copies and rolling out an elaborate marketing campaign with limited-edition hardcovers.

Yarros — who lives in Colorado Springs and has a busy home life, with six children, two dogs, a cat, two chinchillas and a bearded dragon — was exhilarated, but also felt as if she was “in a vise.”

“The pressure was utterly intense,” she said. “I was like, am I going to be personally responsible for dragging down this whole publisher?”

Her publisher assured her it would be a hit. Still, Yarros was unprepared for the frenzy that has erupted over “Fourth Wing” and its sequel, “Iron Flame,” which comes out on Nov. 7.

Since its release in May, “Fourth Wing” has sold more than two million copies globally, according to the publisher. It has been on The New York Times’s hardcover fiction best-seller list for more than six months — with three months at No. 1. It took off in Britain, Australia and South Africa, selling more than 600,000 copies in the United Kingdom and the Commonwealth, and translation rights have sold in around 30 languages.

On TikTok, hashtags for the author and the series have been viewed more than a billion times. Amazon MGM Studios has optioned the series for a TV adaptation, with Yarros as an executive producer.

“It was a slow, steady build, and then it went absolutely mental,” said Rebekah West, Yarros’s editor at Piatkus Fiction in Britain.

The novel’s runaway success stems in part from the boom in romantasy, a hybrid of romance and fantasy that is drawing fans from both genres. “Fourth Wing” is a steamy mix that blends fantasy elements (elaborate world building, an epic battle between good and evil, fire-breathing winged dragons) with popular romance tropes (the enemies-to-lovers plot, plus explosive sex scenes, including one that starts a literal fire).

“It’s just been massive,” said Shannon DeVito, director of books at Barnes & Noble, about the response from readers.

For Yarros, the escalating fame has been jarring.

“I’m not comfortable in the spotlight,” she said recently over dinner in Manhattan, the night before an appearance at New York Comic Con. “I would rather stay home with my kids.”

Navigating best-sellerdom is made even more complicated for Yarros by chronic illness; she has Ehlers-Danlos syndrome, a genetic connective tissue disorder. It can be difficult for her to stand or talk for more than an hour, which makes marathon signing sessions and fan events a challenge.

“Sometimes I feel like part of my job is to make sure she survives this with her health intact,” said Louise Fury, Yarros’s literary agent.

In “Fourth Wing,” Yarros wrote about her condition for the first time, giving her protagonist, Violet Sorrengail, many of the afflictions she suffers from, like dizziness, brittle bones and joints that easily dislocate. Violet’s mother, the commanding general, pushes her to join the elite dragon rider forces like her older siblings, but Violet struggles at the war college. Her condition, which is never named, leaves Violet so weak that she can’t stay on her dragon, nearly plummeting to her death before she grudgingly accepts a saddle that locks her into place. Other dragon riders belittle her as small and fragile, but Violet’s ruthless antagonist and love interest, Xaden, is won over by her determination.

“I read fantasy growing up and I never saw that, I saw these powerful heroines,” she said. “I wanted to tell a story about a girl who should not succeed, and who should not be able to endure an overly brutal environment.”

Yarros grew up as the youngest of four in a military family — her grandfather was a general, and both her mother and father are retired lieutenant colonels. Her family bounced around Washington, D.C., Oklahoma, Kansas, Pennsylvania, Germany and Colorado.

She took up writing poetry and fiction early, and wrote a novel for a high school English project. When she was a college student at the University of Colorado, Colorado Springs, she met her husband, Jason Yarros, a young private in the Army, at a karaoke bar one night. They got married and quickly had a child, and Yarros dropped out of college.

Jason, who flew Apache helicopters, was deployed five times, with four tours to combat zones in Iraq and Afghanistan. While he was gone, Yarros worked on her college degree in history. When she graduated, at age 29, they had five children.

In 2003, after Jason was injured by an antitank land mine in Iraq, Yarros developed insomnia. To occupy herself in the middle of the night, she read romance novels. Several years later, when Jason was on his third deployment, Yarros decided that instead of just reading novels, she would write one.

She knew so little about the publishing world that in 2011, when she wrote her first book, an urban fantasy, she bought “Publishing for Dummies” to figure out her next steps. She signed with an agent, but no publishers made an offer. Undeterred, she decided to write about a young woman in a military family. The result was her debut, “Full Measures,” a romance about a woman whose father is killed in Afghanistan. She sold it to Entangled, and it was published in 2014.

It was the start of a prolific career. From then on, Yarros worked at a breakneck pace, releasing two novels a year. But she often felt discouraged by her stagnant sales.

In the years after she was diagnosed with Ehlers-Danlos in 2020, she thought about quitting. A flare-up had left her so dizzy she could barely walk from her dining room to her couch without collapsing. Her four sons were also diagnosed with the disorder. The stress of writing, managing her illness and caring for her family felt overwhelming.

“I got to a point where I was like, is this worth it?” she said.

Then, in 2022, her pitch for “Fourth Wing” was accepted. She’d written 20 romances, but this was a chance to write fantasy, something she’d wanted to do ever since her first book failed to sell — and to write an otherworldly epic about a heroine with a chronic illness.

“Writing Violet is super cathartic, because she struggles to accept the accommodations that are given to her, and I have that same struggle,” Yarros said.

She wrote “Fourth Wing” on an accelerated schedule, working 12 to 14 hours on some days. Her husband, who had retired from the military after 22 years, looked after the household. When the book hit the New York Times best-seller list, they both cried. “He would kill me for saying that,” she said.

Soon after she was done with “Fourth Wing,” she wrote “Iron Flame,” the second installment of a planned five-book series.

Now, “Iron Flame” is shaping up to be another mega best seller. Barnes & Noble is holding midnight release parties at more than 200 stores, including one at the Union Square store in New York that Yarros plans to attend along with 600 fans; tickets to that event sold out in minutes. Independent stores, as well as bookstores in Britain and Australia, are also throwing midnight parties, a rare occurrence that booksellers say reminds them of the fan fervor around “Twilight” and “Harry Potter.”

At Comic Con last month, a crowd of ecstatic fans gathered at the Javits Center to meet Yarros.

One reader told Yarros she had skipped work to come. Another said his wife had sent him to get her copy of the book signed — and told him not to bother coming back if he failed. Some wore T-shirts and backpacks that said “Basgiath War College.” A few came as Violet, in leather bodysuits with daggers strapped to their thighs.

Several people thanked Yarros for creating a heroine with a chronic illness.

Ashley Sitarski, a “Fourth Wing” fan from New Jersey who was diagnosed with lupus last year, said it was refreshing to read about a character who lives, and thrives, with a chronic condition. “The fact that she wrote her illness into the book is huge,” she said.

Another reader, Evey Alvarez, who has had dire health complications from Ehlers-Danlos syndrome, immediately recognized Violet’s illness, and was gratified to read about a character with her condition who is also “sassy,” she said. “The representation matters.”

The signing was taxing for Yarros, who felt a migraine coming. But she was beaming throughout, introducing herself to each reader, as if she still couldn’t quite believe that they had all come just to see her.

“Hi, I’m Rebecca,” she said over and over.