Lost Tribe of the Sith: The Collected Stories by John Jackson MillerOriginally a series of free eBooks (but with new chapters added in print), Lost Tribe of the Sith tells the story of a group of ancient Sith warriors who get trapped on a remote planet and mix with the locals to form their own culture over a period of generations. It's a companion story to the Fate of the Jedi series, which continues the story hundreds of years later, when the Lost Tribe escapes from the planet and returns to the greater galaxy 40 years after Return of the Jedi.
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A spinoff from the first chapter of this story, based on the Sith party that gets separated at the beginning before the crash, is continued 5,000 years later in Crosscurrent.
This is an Old Republic era book from the Legends timeline. It may be contradicted by newer books, movies, or TV shows (if so, just think of it as an alternate universe).