I need to preface this by saying this is 100% me as an old school D&D guy and Star War fan answering this. The orange tag is doesn't come into play with this one folks. Now, disclaimer out of the way:
-
Lightsabers: Make any magical slashing weapon either be on fire, and just say it's "magic red fire", or look up "Brilliant Energy" (basically fantasy lightsabers from previous editions) weapons. Beware, those were absurdly powerful in older editions. Use one of the Rare magic swords in the DMG as a source item and modify as you will if you want to do it quick and easy. I'm sure if you do some Googling you can find a lot of awesome ideas for 5e Lightsabers.
-
Darth Nihilus: This guy is a Lich. Very, very obviously a Lich. Look them up up in the monster manual and do some homework. If you want to go for authenticity, his Phylactery would most likely be his mask. You're the DM, so you can always bend or adjust the rules (as a phylactery is usually a small box containing something, not a mask). For classes, based on what he does in Kotor 2, I'm guessing he's be a Necromancy focused divine caster (probably Cleric?). I haven't played a Cleric in 5e so don't quote me on that one. Annihilate can be duplicated (sort of) with the spell Power Word Kill. Make sure to use it first on something that isn't a player character so they understand what they're dealing with (and possibly how to preempt or counter it).
-
Darth Sion: As Obo2 said, Bloodrager is a good one. But basically any sort of death defying Barbarian should work. Frenzied Berserker in 3.5 was close. You'd have to flavor his "rage" as "hate", since in the Kotor 2 he is oddly composed for someone mentally holding his decaying body together. Still, a fun and memorable villain.
-
Darth Traya: In Kotor 2 she demonstrates a lot of precognitive and extra-sensory ability. There isn't a 1:1 SW > 5e translation here, but a Diviner Wizard would be close. Again, as the DM remember you can always adjust things a bit to make the game more fun and flavorful. If you want her to have levitating Lightsabers there are a variety of ways to do that like Dancing Weapon, but a more practical way might be to just say that's a property of the weapons themselves (EG: they're magic loot). Just beware what the players will do with them once they (inevitably) get them.
-
Darth Sidious/Palpatine: There's probably a dozen ways people have converted Palpatine into a D&D villain over the years. Based on the abilities he demonstrates in the films, he'd probably be a Wizard or Sorcerer. His defining trait seems to be meticulous planning and ruthless cunning, so incorporate that into your NPC. He is one of the most iconic villains of all time from any media, so consider that your players will likely metagame fighting him once they realize your inspiration.
-
Darth Vader: Like Palpatine, there's probably a lot of examples out there to work from. For me the definitive Vader fight scenes are that Hallway Scene from Rogue One and the fight with Luke in the Deathstar 2. In both cases Vader is slow, methodical, and utterly terrifying. To me, this says "Evil Paladin". I'd make his Smite be a lightsaber throw, and either create spells to duplicate his Force abilities or modify existing ones. Note however that damage over time rarely makes sense in D&D with how fast they've made combat, so Force Choke would probably just be a 1-2 round thing. And like Palpatine, your players will probably realize who they're fighting and metagame a bit. Just remember the point is to have fun and it will all work out.
Also in general, there are a LOT of awesome YouTube video series on running D&D and how to do all kinds of cool stuff. Give it some searches and happy gaming!
devtrackers.gg