Why not play on the pants-wettingly terrifying nature of the Dark Lord of the Sith, as seen in Rebels and Rogue One?
There's a rule called "Inspiring Presence" in the Lead by Example book. It gives one Boost Die per rank to any Cool or Discipline checks to resist fear caused by combat for all allies, as a sort of "look who's here to help us, the odds don't look as bad anymore" thing. Invert the rule, and instead of providing Boost dice to allies, it provides Setback dice to enemies:
Terrifying Presence 3: When this character is engaged with the enemy, all enemies gain 3 Setback dice to any Cool or Discipline checks to resist fear caused by combat.
Then require a Cool or Discipline check every turn to do anything except run or hide from Vader, or maybe a Cool/Discipline check where every Failure translates to one Setback die for any combat action against Vader. Drown them in black dice if they're fool enough to face the Man in Black...
