Somewhere, in the back of his mind, a voice is telling him to sleep. He hasn't done that in a while, he knows; he just can't decide whose voice it is. Is it Darth Sidious, commanding his pupil to be in maximum condition for some battle? Is it his mother, Shmi with all the care and concern that she could give him? Is it Padmé, beautiful Padmé Amidala, because he had another vision of death? Could it even be Obi-Wan, after saying that there was going to be a long day of training tomorrow? Might it even be Ahsoka, excited for the next mission that would come on the morrow?
He doesn't know, and that's what scares him. He, the most despised and feared servant of the Emperor, fallen so far from the glorious days of his youth, is afraid of the voices in his head.
There are a lot of them. In addition to his mother, teachers, student and wife, he can hear the voices of all those he's ever killed; not the least of those is his own, his natural voice, from a time before he was known as Darth Vader. He can hear Anakin Skywalker's voice the most, since it always shouts at him about how he was wrong, how he failed, how terrible and pathetic he is. Then there is Mace Windu, the Jedi Master telling him that he'd likely earned a seat on the Jedi Council and the rank of Master. Count Dooku, Darth Tyrannus, begging for mercy from the young man whose arm he'd taken. The various villains he'd taken down in the name of keeping the peace. Even Yoda, who has proved elusive even to the supreme master of the Force, can be heard from time to time...and, of course, the droids that always followed him everywhere. The beeps and whistles of R2D2, or C-3PO's halting speech, which bring him back to his childhood.
They tell him different things, though it's most often the last words they said to him. As terrible as that is, and as frightened as the voices can sometimes make him, he hates to think that they might one day leave him; they are who he has become, helping to guide him on his path. They fuel his connection to the Force, feeding his emotions in one way or another. When he fights, he fights for all those he lost. He fights for all those he killed. He fights for his mother, for Mace Windu, for Count Dooku. Most of all, he fights for Anakin Skywalker. When he channels his emotions to use the Force, as the Sith always have, he wields it for those he loves. He wields it for those who were important to him. He uses the Force for his mother, for Ahsoka, and for Obi-Wan Kenobi. He wields the Force for Qui-Gon Jinn. He channels the energy of life for his beloved Padmé.
Always, always, always, it comes to Padmé. If he hadn't attacked her on Mustafar, he believed, she would still be alive. With her by his side, he would have gladly taken on the Emperor in a gambit to rule in the man's place...but she's dead, and his anger is what killed her. He often wonders what happened to their children, and whether they will ever rise against the Empire to take down his hated master.
He certainly hopes so. If one of them finds Yoda, or even Obi-Wan, they'll be able to become strong in the ways of the Force. He can then extend his offer of friendship, possibly reconnect with the two Jedi Masters, and tell them that he plans to betray the Emperor. He also wants to rebuild the order of the Jedi, ironically enough, but that could wait until he was able to act freely. Priority one was taking out the Emperor, and he couldn't do that alone.
It's times like this that Vader hates his reality. He lets the voices talk to him, but he hears one of them in particular this time. It's soft and sweet, and he can almost imagine that he's actually hearing it instead of having to picture her while she's saying it to him...and if he has any regrets about his chosen path, the only one would be that she can't be here with him. It's his fault, his problem, and his eternal curse. He has damned himself to this torment; his burden will be to live with it. All of it: the pain, the sadness, the self-loathing, and the hatred towards the Emperor. The memories, the heavy heart, the knowledge that it easily could have been different. The nostalgia, the happiness, the grief and regret. It's times like this that make Vader remember just how much he despises what he's become.
The voice is telling him to go to sleep, but he's not afraid anymore. He knows who it is, with her honeyed voice, that begins to carry him off into the world of dreams. He lays down, eyes closed behind the terrifying black mask that keeps him alive, and he doesn't feel as bad about life as he did a few moments ago. It was always one of the best things about her, that she could brighten his mood whenever she spoke, and her voice still carries that quality as he listens to her speak to him. She whispers words, sweet nothings and verbal kisses, and it suddenly hits him again that she's dead by his hand. Even so, it doesn't diminish the love he's always held for her; if anything, her death has only made him feel closer to her. She's part of the Force now, so he knows that her energy is somewhere in the galaxy.
He falls asleep to Padmé's voice.