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Loking to get back into the swing of things. Crix was a sith, was rescued from a primitive and isolated planet by a darkside cult after he forced a jedi-manner star fighter down and was subsequently defeated by said jedi. He escaped their indoctrination and is now he's roaming the galaxy on odd jobs. Let me know if that's something you can work with! I'm rusty with writing just so you know.
Can you clarify what you're looking for? Are you asking for a master or just threads with dark jedi/fallen sith. Arguably Salis is a fallen sith in the sense she was once sith but now is actually light sided. I also have a falleen sith that's just roaming doing whatever he believes may benefit the sith overall but is doing so rather independently. I do have a second sith, sort of, but he's got a shattered mind with multiple personalities which can take hold, and change his form when they do.
More dark siders though, a witch, a sith apprentice (to not really a named sith but some sith on Dathomir.), possibly a male warlock depending on how things progress for the person who came up with the idea of a patriarchal clan on dathomir. But that one doesn't exist yet.
The witch likely wouldn't teach him much if anything without some real good reason though. And a good bit of certainty she'd be in control. The shattered mind sith, or one of his personalities, might, in exchange for something. But it likely wouldn't be an indefinite apprenticeship and the character is independent. Unlike the first sith mentioned, the falleen, this is not by wholly choice necessarily. But could still be interesting. Alchemy or more standard sith stuff are both things that come up.