Hi! Okay, so, obviously this isn't anything official, but I've seen a lot of people struggle with picking Sith names for either their own characters or, more likely, a Star Wars character that already exists but isn't a Sith in canon (looking at you, Obi-Wan). I've seen generated names, I've seen play-on-words names, and I've seen people just Google "what would so-and-so's Sith name be" or "how to name a Sith", and... Google doesn't really have the best answers (in my opinion), so here is my (mostly) fool-proof way to name a Sith.
Step 1: Open Google Translate, Thesaurus.com, and Etymonline
All of these you can just look up on whatever internet browser is in your vicinity. You might use these, you might not. Thesaurus.com is something I would stake my life on, though, so if you look up nothing else, use this.
Step 2: Look at your character and their personality (or their role)
Alright, so you've all heard the "oh, Vader comes from invader" and "Sidious comes from insidious". That is correct; so your next step is to find the correct word for your Sith. This word needs to embody who they are or what they do, and more than that, what their demeanor and Force presence is like. (This is why I use thesaurus.com lol, cause some of these words are hard to find). You also have to keep in mind that more often than not, it's their Sith Master who names them, so what does that Master see in them and what is their goal for them? If your character doesn't have a Master who names them, treat yourself as their Master and use that same process to find a good name.
Obi-Wan Kenobi- for him, what word you pick greatly depends on how you view him and what characteristic you're emphasizing throughout your story. It could be his ruthlessness when fighting, his manipulation abilities, or his possessiveness towards a certain padawan. For the sake of examples, I'm going to peruse through thesaurus.com and choose the word ravage, because of the way he absolutely destroys his opponents when he's close to the Dark Side (Maul, Anakin, etc).
OCs- my two OCs that I'm going to use as an example are Magur and Salome Caleste. Since they exist only in my brain, I shall cut straight to the point: Magur watches all; he's very observant, so I chose the word vigilant. Salome is very dark, and she enjoys inflicting pain on her victims, so I chose the word sadistic.
Step 3: Letter Arrangement
The final part is purely aesthetic because, you know, Sith are dramatic idiots. All you have to do is take away or add letters until it sounds like... you know, a name. This is where I use Google Translate or Etymonline, because if I'm stumped with a name sometimes looking at the origin of the word makes it easier. You don't have to keep the vowel patterns that your root word had; usually it sounds better if you change them. Note: generally if you want your name to sound more feminine, at "ia" or "a" to the end.
Tyranus (Dooku)- comes from the word tyrrant
Obi-Wan Kenobi- Darth Ravageous
Magur- Darth Illant
Salome Caleste- Darth Adista
Alright, that's it! I hope this helped a little bit... I know it's not much, but that's my method; do with it what you will.

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How To Choose A Sith Name
FanfictionAllegedly. This is how I pick my Sith titles for Sith AUs, OCs, or anything other reason I might need to name Sith. Disclaimer: I do not own Star Wars or the cover art; this is just what my brain came up with, so that's what I'm putting out. :)