Cassandra Clare, the best-selling author of the YA series The Mortal Instruments, announced on Tuesday she’s set to write a new adult fantasy series for Del Rey Books.

The first book, Sword Catcher,is currently in progress and will feature the “criminals, princes, magicians, and warriors” which Clare has been toying with in her head for years. It will mark her first foray into high fantasy and is likely to come on the heels of Queen of Air and Darkness, a Mortal Instruments sequel slated for release at the end of 2018. No publication date information for Sword Catcher has been made available.

“Many of my readers have been with me for years, and have grown along with my characters,” Clare wrote in her Tumblr announcement. “And over those years, readers of all ages have come to me both to say that they would like to read books about older characters, and also to say that they’d love to see me build my own new world from the ground up.”

Clare’s first Mortal Instruments novel, City of Bones, was published in 2007. It kicked off an enormously successful YA series of six books, which has been adapted into both a film and, more notably, Freeform’s TV series Shadowhunters. City of Bones netted Clare a slew of prizes and she’s since followed it up with various series set in the same universe, including The Infernal Devices and The Dark Artifices. Her new high fantasy series, beginning with Sword Catcher, will be entirely separate.