When his boy had left, Yoda had at least expected to hear his name pop up occasionally, considering the title that he had reattached to it. But there hadn't been anything. Not a whisper. So he had assumed that his old apprentice had settled for a quiet life devoid of complication.
He should have known better. Yan Dooku had always had a flair for the dramatic.
The Count steps onto the stage with all of the sophisticated gravitas that he had been known for as a Jedi. He sports the same dark colors he had been fond of then, only now he wears a flowing cape in place of dark robes. When Yan speaks, there is a distinct edge to his words, a cutting precision that dares anyone listening to question his orders, his sentiment, his loyalites.
Yoda confronts him on these grounds first, but he gets nowhere.
"Much to learn, you still have." This spoken after he absorbs the white-hot energy that explodes from his boy's fingertips. No, Yan has nothing to learn in that regard (Yan is possibly the most learned being in the Force after Yoda himself). He speaks these words in a desperate attempt to inspire doubt, but the Count doubts nothing. Not anymore (he only ever doubted the Order).
Yan had been a Jedi once, but no more.
Where there had been light, there is now darkness.
Where there had been warmth, there is now cool indifference.
Where Yan's eyes had been a dark, intelligent brown, they are now…
Yoda blinks.
Yan's eyes are still dark and still brown and they still glint with subdued intelligence that few can match, but there is not even a hint of yellow. Those eyes are not the sick, jaundiced eyes of a Sith who has fully embraced the dark side.
The cave is bathed in green and red as they cross blades: one Jedi and one… Count. Just as Yan had been something of a "dark" Jedi, Yoda now sees a "not-quite-dark" Sith. His boy is somewhere in between the two, and he decides there is hope to be had in that.