A few years ago, Rebecca Yarros almost quit writing. A chronic illness often left her dizzy and exhausted, making it hard to work or even stand at times. She wondered if the stress she was putting on herself was worth it.

Then, she had the idea for a sprawling epic: a romance set at a military academy for dragon riders. The first novel, “Fourth Wing,” became an instant best seller, as did its sequel, “Iron Flame.”

Now, with the release of the third novel in the series, “Onyx Storm,” this month, Yarros has hit a new sales record. The book sold more than 2.7 million copies in its first week. All together, the three novels, part of Yarros’ planned five-book Empyrean series, have sold more than 12 million editions in the United States, according to her publisher, Entangled.

“It doesn’t feel real — none of it does,” Yarros told a packed auditorium of around 1,700 fans who had come to see her in St. Paul, Minn., on Wednesday night.

Print sales alone well exceeded a million copies in the novel’s first week, making “Onyx Storm” the fastest-selling adult fiction title of the past 20 years, according to Circana BookScan, which tracks print sales.

There have been far bigger hits in children’s literature: the seventh and final novel of “Harry Potter” sold 8.3 million copies in its first 24 hours on sale. But in the realm of adult fiction, Yarros’ early sales stand out. The last record-setting adult novel, Colleen Hoover’s 2022 novel “It Starts With Us,” sold 810,000 print copies in its first week.

Yarros currently holds the first three spots on The New York Times’s hardcover best-seller list, a rare feat for an adult fiction series. On Thursday, the series also occupied the first three spots on Amazon’s “most sold” fiction list.

The success of “Onyx Storm” also shows that romantasy, which blends spicy sex scenes and romance tropes with supernatural elements, is not a fleeting trend. Last year, the genre accounted for some 30 million print sales, a rise of 50 percent over the previous year, according to Circana.

When Yarros became a fixture on the best-seller list, with the release of “Fourth Wing,” she had already published around 20 contemporary romance novels. But sales from book to book were largely stagnant, and she struggled with a chronic illness, Ehlers-Danlos syndrome, a genetic connective tissue disorder.

Her illness, though debilitating at times, inspired her to write “Fourth Wing,” she said. Yarros grew up loving fantasy, but had never read a fantastical novel with a protagonist who had physical limitations like she did. She decided to write about a young woman named Violet, who enrolls in an elite military academy for dragon riders, and is determined to succeed despite a chronic illness that makes her weak and physically frail.

When “Fourth Wing” came out in 2023, it arrived at an ideal moment. BookTok was in the middle of a romantasy craze, and fans quickly coalesced around Yarros. In the months after the novel’s release, videos with hashtags for Yarros and the series were viewed more than a billion times.

The genre’s growth has been driven largely by the authors Yarros and Sarah J. Maas, whose hugely successful series, “A Court of Thorns and Roses,” helped make her the best-selling author in the United States last year, with sales topping 7.7 million copies. Globally, English-language editions of her books have sold more than 55 million copies.

Some booksellers say the fervor around writers like Yarros and Maas, whose fans turn out for midnight release parties and dress up in elaborate costumes, is something they haven’t seen since the days of “Harry Potter” and “Twilight.”

“Fandom, in the past several years, does have this very passionate parasocial element, where people feel very connected to the creator,” said Annie Metcalf, the events and marketing manager at Magers & Quinn Booksellers, an independent bookstore in Minneapolis that hosted Wednesday’s event for Yarros.

Fans’ fervor was palpable at the event, which was held in a huge auditorium in St. Paul. Many in the crowd were dressed up in sweatshirts, T-shirts and hats that said “Basgiath War College,” the military academy of the series. Some were fully decked out in costume, dressed as Violet, in leather body armor, or as dragons.

“People were handing out friendship bracelets like it was a Taylor Swift concert,” Metcalf said. “It’s wild.”

Waiting outside the theater before the event, Kaylin Lu, who was dressed as Broccoli, a cat character in the series that has gained its own fan following, said she wasn’t a romantasy reader until she picked up a copy of “Fourth Wing” when she was working at Barnes & Noble. She quickly went from agnostic to evangelist. “I was like, You know what, that book looks fun, I’m going to try it out,” she said. “And then I was like, Everybody, listen, everybody must read this book.”

She pressed the book on her friend, Diane Nguyen. She was also hooked, and had joined Lu for the event, dressed up as a dragon. “There’s action and romance and it very fluidly moved together,” Nguyen said. “Thanks to her, I love reading again.”

Inside the theater, as the boisterous crowd waited for Yarros to appear, Alejandra Herrera sat in the front row, almost vibrating with anticipation. She had driven some 380 miles from Omaha to attend the event, she said.

“I’m a little nervous,” she said. “We’re going to make eye contact with her.”

Herrera’s friend Rachel Peterson, who also made the drive from Omaha, said she came not only to see Yarros, but to gather with other fans. “It’s nice being in a space with so many other people that love the series,” she said.

At the start of the event, Yarros was introduced by the romance writer Abby Jimenez, who noted that the Empyrean series had taken over the top three slots on the New York Times best-seller list.

Yarros uttered an expletive to express her surprise and gratitude. “Thank you,” she said to the crowd. “You guys did that.”

Jay Senter contributed reporting from St. Paul, Minn.