Cataclysm is the third (in chronological order) main storyline, adult canon novel in the Star Wars canon material. It is set during the early High-Republic era and picks up shortly after the events of the previous book, Battle of Jedha. The story is still set around the warring neighboring planets of Eiram and E'ronoh. The Jedi discover that "The Path of the Open Hand", which is basically a cult that appears to be set up as the precursor to the Nihil who are the antagonists in the later High-Republic novels, sabotaged the peace talks on Jedha and are using the warring planets as a part of the larger plan to take out the Jedi. The climax of the story occurs on a planet called Dalna, where the Path has a stronghold, and a massive battle between the Jedi and the Path occurs.
This book finally does what I have been waiting for the High Republic novels to do, namely incorporate Yoda, the most recognizable Jedi alive at the time, into the larger storyline. In all of the other novels, be they the adult novels or the young adult novels, if Yoda is mentioned, it is in passing and amounts to a sentence or two. In this book, he actually appears and takes part in the story. Actually, he and Yaddle both appear, and Yaddle has an even larger role in the story.
The book is a good blend of action and suspense. It reveals more about the mysterious entity that can block the Jedi's use of The Force and turn people to ash (which introduces a bit of a plot hole in the later High Republic novels in which the Jedi seem totally clueless about what it is), and the author does a good job of making you care about and get invested in the new characters, having used the two previous books to establish those characters. The other big issue I had with the first batch of High Republic novels (which occur later in the chronology than this book) is that it seemed like every book introduced a bunch of new characters and it would take three books to return to the storylines of previously introduced characters. And by then, it was hard to keep them all straight. While this book does introduce some new characters, many of the characters that were introduced in the books Convergence and Battle of Jedha also appear in this book, so it feels like a cohesive story.
In my opinion, this is the best of the High-Republic novels (at least so far), with the proviso that I have read all of the adult novels and most of the young-adult novels set in the High-Republic era. The story flows well, it wraps up the story told in the first two books and provides almost a blank slate from which future books can pick up. The book is just under 400 pages and can easily be read in a week or so, depending on how fast you read and how much time you have to read. It is definitely on my must-read list for die-hard Star Wars fans.