Luke dies in his fight with the Emperor before Vader has a chance to save him.
This breaks Vader, to the point where he just fucking leaves the Empire and goes off to find Leia “there is another” Organa and what remains of the Rebellion.
And he shows up on Leia’s doorstep, to which Leia shoots him immediately. “Okay that’s fair,” he says, wheezing and holding his wired shoulder in pain.
“What do you want?” she says, this time aiming the blaster at his head. “Answer quickly.”
He tosses her Luke’s lightsaber. “This should belong to you. He would want you to have it.” He coughs, “And I want to join the Rebellion.”
Leia lowers her gun very carefully. “Explain.”
“What is there to explain?” Vader stands fully, towering over Leia. “He killed my son. I want the Emperor dead. At this point, I no longer care what government replaces him so long as the man who murdered my son is dead.”
Leia thinks I can work with that.
So you have Leia here steadfastly ignoring Obi-Wan and Yoda’s ghosts who follow her around spouting Jedi nonsense, of which she refuses to listen to unless they let her talk to Luke, who instead turns to Darth Vader to learn the ways of the Force. “Lost, all hope is” except it isn’t.
Because you have Darth “Probably Not A Dark Sider Anymore But Also Wouldn’t Call Him a Jedi” Vader and Leia “Anger Is My Middle Name But I’m Also Firmly In the Light” Organa teaming up and wrecking havok on literally everything. Seriously, there are whole planets on fire.
Vader teaches Leia the Force, and they never bring up the fact that they are related if they can help it (which means, inevitably, that they end up talking about it occasionally: “You look like your mother,” “…What was she like?” v. “I see now why Obi-Wan didn’t train you. You couldn’t be more my daughter if you tried. Poor Bail…I can only imagine what you were like as a child.” with Leia sputtering “HOW DARE”).
meanwhile Han Solo is following them holding a baby Ben (who, I imagine, is now named Luke) going “guys? guys? i don’t understand. why is darth vader here. leia why haven’t you shot him. leia can I shoot him? leia we should probably not set this planet on fir–okay, so we’re setting this place on fire. cool cool cool”
in the afterlife, Obi-Wan and Yoda are mourning about how ALL IS LOST while Padme and Luke are drinking mimosas and laughing because they knew there was good there all along.