What is romantasy? The best-selling book trend, explained.

It’s been awhile since a single book could summon thousands, but in November and again in January, fans lined up outside bookstores in the cold, awaiting the stroke of midnight. The draw? Romantasy.

Rebecca Yarros’s “Iron Flame,” which broke a preorder record, arrived on Nov. 7 and shot to the top of bestseller lists worldwide. Then came Sarah J. Maas’s “House of Flame and Shadow,” the third volume in the Crescent City series. Released on Jan. 30, it sold more than 120,000 copies in the first week and was an instant bestseller too. Maas’s biggest competition may be herself — she has four novels on the New York Times bestseller list and 13 on the USA Today list — and Yarros, whose books are on five bestseller lists.