His injuries from Malachor ache in places he long thought dead to feeling. His dreams, when he can sleep, return to Mustafar and to fire and to loss. No surprises with that. His own mind has always been the greatest traitor to his own ends. Malachor reopened old wounds, seeing the face of his failed apprentice, and added new wounds as she attacked him with no mercy in the name of all her own losses. Vader has killed hundreds who once called Anakin friend. One more should be nothing.
The suit keeping him alive has a feature to allow painkillers when he requires them. He disabled the function years ago. Pain focuses him. Pain sharpens his anger. Pain drives away the softening moments inside him when his dreams linger too long on happier times.
Too much pain, on the other hand, clouds his mind and brings on mild hallucinations. That is the only explanation he has. He stands at a meeting, annoyed with the incompetence surrounding him and eager to break away. Across the table, the colors of a chair, a uniform, and the bland, gray wall trick his eyes into seeing an outline that cannot be there. Vader is failing to pay attention to the dull reports being given, while a shape that looks exactly like the woman he killed stares at him.
He makes an excuse and strides out, leaving her there.
Ahsoka is dead. He is positive he left nothing but a corpse behind him in the ruins of the Sith temple. He felt the give of her flesh as his lightsaber cut through.
His mind is playing tricks on him. Perhaps the optics in his face mask need adjustment. Perhaps he requires more rest from his experience. He finds a reason to excuse himself from other duties for the remainder of the day and goes to his own chamber for meditation.
Ahsoka waits for him there, seated as though they have made some appointment.
Even as he approaches, dread in his gut, she becomes more clear, although not solid. Her own wounds are gone, except the wounds in her expression as she watches him.
"Go away," he says out loud, aware that this is a sign of madness.
She shakes her head.
Vader makes a fist. The empty gesture only draws a far more familiar expression on her ghostly face, one of annoyed patience wearing thin. He is the Master and she was the Apprentice, yet she is the one sitting in judgment of him, disappointed that Anakin hasn't done better with his life.
He is no longer Anakin. Ahsoka is not his concern. Could he feel guilt, he still would not care. Darth Vader has committed far worse acts to encroach on his husk of a conscience than the slaying of one would-be Jedi.
Vader turns away from the apparition, and kneels in his own silent contemplation. The pain gradually fades from his awareness. The room drops away. His mind is at one with the sable night, enriched by dark power and by the clarity of his honed angers. He will not be chained by the past.
He emerges from his trance, and Ahsoka is in the same position, still watching him.
"What do you want?!" he shouts at her.
She tilts her head. "The dead want nothing, Anakin."
He stumbles back. The words were clear, exactly her voice, echoing through the room and inside his skull. He grabs the nearest item to hand, the hilt of his own lightsaber, and hurls it at Ahsoka. It passes through her body, clattering against the chair.
"Leave," he says, with a desperate plea.
"I'm never leaving you again."
Vader storms out of his quarters in terror for his own sanity. He makes his way to the bridge of the Star Destroyer, shoving underlings out of his way with the Force. They have paused in this group of ships for the meeting he is not attending. "Captain," he orders, "take us to hyperspace."
The Captain's face is momentarily confused. "On what course, Lord Vader?"
"Coruscant." He's chosen it at random. The direction doesn't matter, only fleeing does.
He feels a relief which he's glad will never show on his face when the stars warp around the ship. Explanations can come later for why he ran.
"You always did love to fly," Ahsoka says, standing beside him and watching the blue radiation cascade in their wake. Vader turns to her, notes that no one else here sees a phantom Togruta woman or hears her amused voice. Speaking to her will only spread rumors of his mental disarray. He opts to ignore her as she finds a comfortable seat on the Bridge, kicking her feet in a bored fashion as they wait.
He should contact his Master. He should ask how to rid himself of this incorporeal pest. He should research if this has happened before.
He continues to ignore her. Ahsoka walks at his indifferent elbow while he marches down the corridors of this ship back towards his quarters. She will not leave his side. The gleeful mirth he catches on her face as he enters his chambers tells him she intends never to leave his side. Sleeping, waking, he has called her spirit from the Force, and here she will stay.
He sits at his desk. There is work to be done, and a report to write for why he abandoned the meeting in a rush. He doesn't have to listen as Ahsoka walks around his room, taking a look over his few possessions and commenting on each one. "Why do you have this? This book is terrible. You used to have better taste. And the shelf is covered with dust. Don't you ever let the cleaning droids come in here? You were always messy."
"I am working. Stop talking."
"It wouldn't hurt you to pick up a broom, Anakin."
"Shut up." It's immature, and beneath him, but Snips always brought out his inner adolescent.
She sits in another chair, observing him as he creates his report. "You spelled that wrong."
"If you were not dead, I would kill you again."
"If I wasn't dead, we couldn't be spending this time together."
Frustrated, he shoves his computer away. He surrenders. "You want something. Tell me."
Ahsoka stands and approaches him. He can see the far wall through her body. His heartbeat is regulated by his suit, and cannot race faster as she nears. His breathing cannot quicken as she reaches out her hand and touches the smooth surface of his new helmet.
"I want Anakin back."
"He's dead. As dead as you are." Vader should know. He murdered them both.
"Then I will remain here with his ghost until he comes home."
Her arms reach out to him. He wonders if she has enough strength now that she has rejoined the Force that she could kill him. Part of him, that part she's seeking deep inside his burnt soul, hopes she succeeds. His last victim will stretch out her power to obliterate the monster in one last burst of ethereal fire. Instead of pain, he feels the impossible, intangible touch as Ahsoka embraces him.
"I'm here," she says. "I will always be here."
After a long moment, he reaches up one arm and lets his mind believe he can feel her as he holds her, too.