The name Darth Vader means nothing to him, but something about the turn of those syllables feels like a klaxon going off. Like he should know it, like it's important. What it suggests, though, is that this Master is cycling through apprentices. It isn't Dooku, or Maul, or Ventress — it isn't this Vader. What is of import, though, is the hesitation in Ezra's answer. The way he carefully tiptoes around the answer.

It's with a sinking sensation that Obi-Wan says: "I've already met him, haven't I?"