The Dregs of Empire is a novella set in the Sun Eater universe, and in my opinion is the first that is truly required reading, and also the best of the non-main novels.

It's also the first side story that can't be read in any order. If you haven't read up through the fifth main novel, Ashes of Man, do not bother reading this one as it will make no sense. Dregs of Empire follows Lorian Aristedes to his exile on the frozen prison planet Belusha after the events of Ashes of Man, and features only his POV. Lorian is one of my favorite supporting characters in the main series and one of the most developed, and Dregs of Empire continues his character development and contextualizes some his actions and thoughts in the main series.

The first half of the novella is a bit slow, describing the day-to-day drudgery of a camp prisoner, but it quickly ramps up and Lorian's skills that are mostly off-page in the main series are on full display here. The "physically weak character that is actually a tactical mastermind" trope is a bit tired, but Ruocchio pulls it off well here, and Lorian is an easy character to root for.

Dregs of Empire does a great job of portraying Lorian as much more than just Hadrian's tactician; he's a good man that is also very, very flawed, and here we see more of him than the usual wit and sarcasm (though it is present here as well). Very interested to see when or if Lorian rejoins the main series. It would feel like a waste for this to be the end of his story.