The hotly anticipated follow-up to the bestselling books "Fourth Wing" and "Iron Flame" was released Tuesday, accompanied by midnight release parties and retailer website crashes.

"Onyx Storm" is the third in the "Empyrean" series by author Rebecca Yarros. It's credited with fueling the popularity of a new genre dubbed "romantasy," combining romance and fantasy themes.

The books are set in the world of a war college for elite dragon riders, where characters fall in love while fighting to survive.

The books are available at independent bookstores and big-box retailers, but a special deluxe edition of "Onyx Storm" was available as a Target exclusive.

The exclusive edition features "sprayed edges with stenciled artwork, as well as exclusive special design features," according to the item listing.

Fans of the series reported that the Target website experienced outages overnight as readers rushed to order copies. The website Downdetector.com, which tracks online outages, received hundreds of reports between 1 and 4 a.m.

Readers attended midnight release parties at Barnes and Noble stores in Eden Prairie and Edina, braving subzero temperatures for a chance to celebrate the beloved series.

Stores offered trivia and other games ahead of the book's release.

As of 11 a.m. Tuesday, the Target exclusive edition was sold out online.