We know a bit about Ren's backstory of course, but not really a chunk of the details, except that things probably went bad about five years before the sequel trilogy, where Ben found out the truth about his grandfather due to a complicated plot (The novel "Bloodlines"), and of course the details in the films that he was somehow corrupted by Snoke, nearly killed by Luke, destroyed Luke's academy and joined the First Order. Presumabely details about what exactly the Knights of Ren are will also be filled out in Rise of Skywalker.

There's still a fairly big canvas to play with in the thirty years between the trilogies, including more of Ben/Ren's story. Wonder if this novel will cover some of that?