Luke Skywalker had given himself up to Darth Vader so he could get face to face with Emperor Palpatine. Once he was on board the second Death Star, he got to see Palpatine's face. Noticing what appeared to be facial scars, he said, "What happened to your face?!"
"I got old. I was an old man even twenty five years ago," the Emperor replied.
Vader said, "I saw you use Force lightning on a Jedi that deflected it back at you with his lightsaber!"
"Shut up, Vader,"
Luke watched the two of them argue. When Palpatine noticed Luke again, he said, "Vader! Fight your son!"
Vader attacked Luke. During their battle, Luke got the high ground. Remembering what happened when he tried to attack the late Obi-Wan Kenobi from the low ground, he threw his lightsaber at the supports for the platform Luke was on, causing him to fall. The battle ended when Luke amputated Vader's right hand. Vader screamed in pain and said, "Why did you have to sever my robot arm that replaced the severed hand from so many years ago?!"
Luke realized that he was about to fall to the dark side. Palpatine said, "Kill Vader. Kill him now!"
Luke said, "Screw you! I know what he's been through since he did the same to me!"
"He didn't lose every single one of his organic hands and feet!" Palpatine said.
"I'm not going to kill him,"
"Fine. Be that way. If you won't be turned, I'll just kill you!"
Palpatine used Force lightning on Luke. Luke screamed in pain and something awoke in Vader. He picked up Palpatine and threw him into a chasm that he couldn't see the bottom of. As he fell, Palpatine said, "I'll be back!"
But as if to call his bluff, an explosion of Force energy came from the pit. Vader felt pain in his chest and said, "Oh crap, the Force lightning disabled some of my life support! I'm going to die now!"
Luke went up to him and said, "What can I do?!"
"There's nothing you can do to save my life. But maybe you can remove my helmet so you can at least see my scarred face before I die,"
Luke did so and, while expecting scars, didn't expect what color his skin had become. Vader tried to think of last words, but keeled over before he could say any more. Luke, remembering that the Rebel Alliance was attacking the second Death Star, got Vader's corpse onto an Imperial shuttle and took off before it was destroyed.