All emphasis in the following quotes is mine.
The Emperor didn't tell Vader that he had captured Luke, but Vader finds out and he is pissed:
- HAD ABBADON – THE TOMB’S PRISON
Luke lies unconscious on a small island in the middle of a hot lava lake. A hot wind blows across his face and wakes him. He looks around at his hostile surroundings and suddenly becomes aware of another person sitting behind him. He turns on the intruder with Jedi speed. The shrouded figure doesn’t move or even look up.
LUKE
Ben...
Slowly, the hooded figure looks up, revealing his face. It is Ben Kenobi in the flesh.
BEN
I could no longer stay in the netherworld.
LUKE
But Yoda said...
BEN
...that I would become one with the Force and choose not to return to the material world? That your anger prevented me from...
LUKE
But I’m not angry... not anymore... I understand why you didn’t tell me about my father. I do.
BEN
I know. And I am here to help you destroy the Emperor, and... your father.
LUKE
I can’t.
Suddenly Yoda appears beside Ben.
YODA
You can and you will... I in the netherworld, and Obi-Wan at your side. Help you we will.
Luke stands with a start and begins to look around. Ben slowly stands also.
LUKE
Did you feel that?
BEN
Yes. It’s the Emperor. He’s moving this way...
LUKE
...and so is my father.
BEN
He is following the Emperor. Yoda!
YODA’S VOICE
I am between them and the Force... cloud their minds, I will.
BEN
Luke, you must take them on one at a time. Together, they will destroy you...
The Emperor enters the cavern and looks across to the boiling lava lake. He sees Ben and is surprised.
EMPEROR
So you are not dead after all, Obi-Wan. I thought I felt your presence in the netherworld. So, you have returned. But you have failed... the boy is mine, and will soon embrace the Dark Side.
YODA
It is you who have failed, my old friend1.
The Emperor turns around to see the shimmering image of Yoda standing before him.
EMPEROR
You!
YODA
Evil one, this will be the last day of your existence.
EMPEROR
Your presence in the netherworld can do me no harm. And neither can this boy or this old man.
YODA
Your fear is strong... overestimate your abilities, you do.
Vader steps out of the shadows and stands next to the Emperor.
VADER
My Master, they must be destroyed. Now it is too late to turn the boy.
Luke looks at his father, then reaches out and Vader’s lasersword leaps off the Dark Lord’s belt and into the young Jedi’s hand. Luke ignites the lasersword as the Emperor pulls a sword out of his sleeve and tosses it to Vader.
EMPEROR
Destroy them.
Vader’s sword ignites and he starts to move toward his son. Luke backs away.
Vader brings his lasersword down hard on his son, but Luke is able to block his father’s blow. A quick sword exchange, and Luke forces Vader back. Another exchange, and the Dark Lord recovers, pushing the young Jedi onto a small rock surrounded by the bubbling pool of lava. Ben and the Emperor watch from the far side of the cavern2.
EMPEROR
He is every inch his father.
BEN
He is stronger than you imagine, and has many allies.
EMPEROR
Allies? If they are all as you, I have no concern. Once the boy has killed his father, I will have him destroy you...
BEN
Perhaps, but it will not save you. I have foreseen your death at the hands of Skywalker.
EMPEROR
But I have not, therefore it cannot be.
BEN
Unless you can no longer see…
Luke fights back with all of his strength and drives his father back across the tiny rocks. A crashing blow by Luke causes Vader to lose his balance and fall onto one of the small rock islands, his sword hand landing in the molten lava. His sword and part of his metal are melting away. Vader backs away from his son, expecting the death blow at any moment. But Luke hesitates as Ben and the Emperor watch.
EMPEROR
Finish it, boy! It is the power of the Dark Side that you feel...
Luke lowers his lasersword.
LUKE
I cannot. I will not turn to the Dark Side. I only fight in self-defense, not to do your Emperor’s bidding.
With that, Luke extinguishes the lasersword and tosses it to Ben.
LUKE
If he is to be destroyed, you must do it3.
Lord Vader struggles to stand as Ben prods the Emperor onto the same small island. Luke stands behind Ben and watches.
EMPEROR
You have become old and weak, Lord Vader. I will take care of this.
The Emperor turns his back on Vader and faces the old Jedi, raising his hands in the air, which causes lightning to strike at Ben4.
LUKE
BEN, NO!
Luke leaps in front of his mentor, and raises his hand in defense. The bolts are deflected by an invisible shield. Wherever a bolt hits, the image of Yoda momentarily appears at the point of impact.
EMPEROR
Your Master in the netherworld cannot protect you for long. You will turn to me or die.
The lightning bolts become more intense, and Ben is knocked unconscious. Luke drops to his knees under the pressure.
EMPEROR
This is the power of the Dark Side that you deny. Your strength will never match it.
Luke struggles to remain conscious against the superior power of the Emperor.
LUKE
Yoda...
EMPEROR
Obi-Wan foresaw my destruction at your hands, young Skywalker, but it seems his vision was clouded... Perhaps there is still another Skywalker. Why can I not see, could the netherworld have influenced my perception? Another Skywalker... your father!
The Emperor turns around to see Lord Vader flying at him. The lightning bolts around Luke disappear as Vader hits the Emperor, knocking them both into the fiery lake of lava. The hideous screams of the Emperor are soon muted. Luke struggles to his feet and stares at the spot where his enemy and his father disappeared into the cauldron of molten rock.
Ben puts his hand on the young Jedi’s shoulder.
BEN
It is in Yoda’s hands now.
[“In the rough draft... Ben explains that... if Vader becomes one with the dark side of the Force, he will lose all identity. If he turns to the good side, he will pass through the Netherworld and” in the revised rough draft, Yoda “will rescue him before he becomes one with the Force.” -L. Bouzerau, explanation from Star Wars The Annotated Screenplays page 300.]
LUKE
He turned back to the good side.
BEN
Yes, he did.
The hallway is deserted as Luke and Ben wander, sadly thinking about what happened. The rumble of distant explosions is heard.
BEN
It is over...
[NEXT-TO-LAST PAGE MISSING – DETAILS IN FOLLOWING PARAGRAPH]
[“In the revised rough draft (Scene 138, Luke talks to Ben, and then)... Yoda suddenly appears as flesh and blood. His stay in the Netherworld has been resolved since Vader has turned to the good side. Yoda also has been able to prevent Vader from becoming one with the Force...” -L. Bouzerau, explanation from Star Wars The Annotated Screenplays page 320.]
Quietly watching the festivities from the side is Yoda, the Jedi Master. He scans the crowd picking out Artoo, Threepio, Lando and Chewie, Han and Leia, and finally Ben, Luke and his father. He lets out a great sigh.
- Jedi Bendu Script Site, Revised Rough Draft of Revenge of the Jedi