Darth Vader was in the battle arena--or, at least the vicinity of the battle arena, giving a glowing blue holo-projection of himself a pre-duel pep talk:
"I am going to lose, am I not? My opponent is my son, Luke Skywalker," said the holo-projection, which seemed to be malfunctioning at the moment (to Vader).
"I find your lack of faith disturbing," announced the real Darth Vader.
"You are right. If he gives in to his anger and strikes down my Master, the Emperor, then his journey to the Dark Side will be complete!" The projection quivered with triumph.
Vader agreed. "The ability to destroy a planet is insignificant next to the power of the Force."
He and the projection both gave a simultaneous, evil laugh.