The sixth book in the Old Kingdom series is likely to be released in October 2021. It is a prequel to Sabriel.
According to Garth Nix’s Twitter account, the short story A Wind from the North has been subsumed into this book. 
Plot Summary Edit
The story will follow Sabriel’s parents Terciel and Elinor. 
- ↑ https://twitter.com/garthnix/status/1279040593792921601?s=21
- ↑ https://twitter.com/garthnix/status/1217663879733317633?s=20