"Father! Please!"
The lightening flashes. It's blue and white and strong; you can tell just by looking that it is pure, unadulterated power. It pours from his master and crackles with the Dark Side.
Luke writhes on the ground, face twisted in agony. He's screaming and convulsing and–
{"Anakin, help him!"}
He doesn't know why but another face, also tormented by pain, flashes through his mind. She's calling to him and screaming and though it was only a dream, the memory is as sharp as a blade.
Luke's still yelling, and the power is still blasting from Sidious and Vader's still… doing nothing. He's standing silently at his master's side, as he's done all these years.
But he can feel Luke {and a whisper of her} in the Force– feel his pain, and his realization that his father is just standing by and letting this happen.
It isn't the first time, though, that he's stood by and done nothing when he should have.
For a moment, just for a moment, he's 23 again, and Mace Windu is holding his blade aloft, fending off the same attack being used on his son. And for a moment, he stood by, wracked with indecision. But then he did decide, and his blade flashed through the air, and then Windu was falling and he wanted to say, "Wait, wait I haven't decided who I am yet–".
Then it was decided for him and he was Darth Vader and Anakin Skywalker was gone.
{He'd only wanted to save her.}
Now he blinks and he's back in his half machine body, and it's his son being attacked by Sidious, and he's still doing nothing.
The last time, he waited too long and by the time he'd reacted, he'd lost everything; who he was, his wife, his brother, his future, everything. Now he was losing everything again. (Which was odd, he hadn't realized he'd had anything left to lose; but one was called Luke and the other Leia)
It was those that made the decision he hadn't even realized he'd been making. Her face from his dream, his son's face from this reality, and the unreachable daughter who was somewhere in between.
He wasn't going to lose it all again because he stood on the sidelines, wracked with indecision.
{Now all he wants is to save him.}
It almost feels like breaking the surface of water and taking a deep breath; it's like jolting from sleep and looking around and thinking, "I'm awake"; it's like coming home after a long absence, closing the door and calling, "Padme? I'm here."
Anakin Skywalker looked from sith lord to son, realizing his time was running out. And Anakin Skywalker made the decision that Vader never could.
"NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!"