Thrawn Ascendancy: Lesser Evil was an amazing wrap-up to the Thrawn Ascendancy trilogy, and a great lead-in to the Thrawn trilogy that Thrawn Ascendancy is a prequel for. I really liked how it tied up the arcs of the cast of characters and solves some of the many mysteries about the Chiss Ascendancy.
Spoilers ahead for Thrawn Ascendancy: Lesser Evil by Timothy Zahn!
Wrapping up storylines was definitely a big task for this novel as there were so many subplots going on- from Thurfian and Zistalmu's alliance to take down Thrawn, to Thalias and Che'ri's skywalker business, to the conflict with the Grysk behind everything and more. Yet I think it did everything very, very well. All of the characters ended up in places that made sense for them and felt right for their characters and like a natural conclusion to the story. Zistalmu and Thurfian's tentative alliance fractures as Thurfian becomes Patriarch of the Mitth and needs to focus on other things besides just taking down Thrawn, and the Grysk limp away from a battle where Thrawn gathers Chiss forces on the goodwill of his friends and allies.
Learning more about the sky-walker program was also a highlight of this book for me. We meet Thrawn's sister from his original family, though she has no memory of him, even though he remembers having a sister that was taken from him when he was young. She was a sky-walker, and we discover through her that the Ascendancy's sky-walker program erases the early memories of these young girls during their training so that they can better focus on their roles as navigators. Thalias is troubled by this discovery, and there's actually a sad scene where she reveals to Thurfian that she knows this secret as leverage to get him to help Thrawn- but he points out that if it was released to the public, people wouldn't care as much as she thinks they would and it would hurt the Ascendancy long-term if they had less sky-walkers. It's a really sad scene because you can feel how hurt Thalias is when she realizes that he's right, and that people aren't as compassionate towards the sky-walkers as they should be despite their importance to the Ascendancy.
Flashbacks with Thrass were another part of the book I really liked. Most of the "Memories" chapters of the book were used to look back over the relationship between Thrawn and Thrass, Thrawn's brother who died before the series. We learn about how they met with Thrass welcoming Thrawn into the Mitth family, and how their bond grows as they solve problems together and work together over the years. It was really nice to get to know this character, even if we know what happens to him, and to see Thrawn have such a brotherly connection with someone.
Finally, the book leads in perfectly to the first Thrawn trilogy. It ends with Thrawn being exiled, as he said he was in the first Thrawn novel. But as we also know, just being exiled isn't necessarily the whole truth. Thrawn has heard rumors of the Empire, and wishes to know more about them, so his exile is part of a plot to gather information and report back to the Ascendancy. He thinks he'll be gone a year at most, but we know from the Thrawn novels and Rebels that this obviously isn't what happens. His exile is ordered by the Ascendancy's Ruling Families, but the Expansionary Defense Fleet turns it into this plot, on Thrawn's suggestion. It was a great way to wrap up the series and tie into the next series of events, and I loved getting to know all the details of how exactly this previous chapter of Thrawn's life came to a close.