Lost Tribe of the Sith: The Collected Stories

Lost Tribe of the Sith: The Collected Stories by John Jackson Miller
Originally a series of free eBooks (but with new chapters added in print), Lost Tribe of the Sith is a prequel to the Fate of the Jedi series set 40 years after Return of the Jedi. It 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.

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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).