Lord Vader would have to be punished.
Usually, this was accomplished easily enough—a bout of Force Lightning directed at what was left of the man's living tissue, or a short jaunt through his apprentice's mind to tear out one of those offensively persistent memories. Today, however, Vader's treachery had gone too far, further than even the expected amount that occurs between a Sith Master and his apprentice. He would have to be more than reprimanded.
He would have to be taught a lesson.
Palpatine had sensed that the anniversary of Amidala's death would negatively affect Lord Vader, that his tendency to sulk and lash out in turn would be even greater. The extent of Vader's tantrum had been hidden from his sight, it seemed, as his apprentice had actually abandoned his fleet to visit the late senator's mausoleum- and take her body hostage within it. He'd been there for 36 hours after smashing the glass casing and removing the corpse; and it wasn't until a Nubian official worked up the nerve to appeal to Palpatine that he could be removed. The process had been embarrassing enough- Palpatine himself had to leave Imperial Center to fetch Lord Vader, and when he'd arrived to take in the scene- Amidala's limp body, her funerary makeup smudged—Vader actually raised his lightsaber to his master before being disarmed.
All of this could be forgiven, perhaps- a light punishment, with Vader tasked with the humiliating process of cleaning up his own mess and silencing any loose ends- had his wayward apprentice not committed one unforgivable act of treachery. Palpatine had pushed his clone escort aside, coming to Lord Vader's side to make sure the stupid boy didn'tdo something even more idiotic, but when he laid his fingers against the fool's deeply scarred forehead when a single shriek tore from Vader's throat—
"MASTER!"
But the image in his mind was not of his true Master, the one who had given the power of the entire Galaxy to a foolish young Jedi and made him Lord of the Sith. No, for a brief moment Palpatine was almost knocked off his feet by the strength of his apprentice's call for Obi-Wan Kenobi.
Unforgiveable. After all the work he had done for the boy. After all the time he had spent undoing Kenobi's influences, piecing the broken meat of Anakin Skywalker into something usable, the boy still harbored some kind of affection for the Jedi.
He would have to be punished.
Palpatine had genuinely hoped he would not have to use this device, a torture chamber made specifically with his apprentice in mind. In truth, 'torture chamber' was too clumsy a term- it conjured up images of barbaric devices that were more Vader's realm of use than his own. No, this was a room for specifically designed with…reprogramming in mind.
Palpatine watched idly through the one-way transparisteel as his apprentice, stripped of his armor and with only a breath mask, crouched in the middle of the darkened room. His view of the man was assisted by the night-vision enhanced viewscreen, to Vader, the room would be totally black. He knew by now the walls and floor were increasing in temperature to an unbearable degree. To his credit, Vader didn't call out with apologies or explanations, nor did he reach along their bond. He simply crouched, waiting. Palpatine decided he had waited long enough.
With a press of a button, a gulf of flame shot through the air, missing Vader by millimeters. The man shrieked, leaping into the air, and scrambled into a corner, his back pressed against the scorching walls. Another jet of flame erupted, and this time it caught Vader across his right shoulder, neck, and face. The bond between them exploded with agony, and Palpatine shivered with pleasure as a wave of terror swept throughout the base. Even those not sensitive to the Force would feel a twinge of fear. Vader's mental state had thoroughly degraded. Just one more push…
A quick succession of flames, some catching Vader as he tried to crawl away, was enough to send him over the edge. The man's terror turned to blind panic, and Palpatine could see the reason leave his eyes. The memory of Mustafar was never far from Vader's conscious (Palpatine had made sure of that), and now Palpatine brought it roaring to the surface.
He would make Vader relive his rebirth.
Vader howled, slapping at his flesh and writhing in an attempt to extinguish flames that weren't there. New burns widened and deepened as he tore at them. After a while Palpatine found his screams irritating, and shut off the amplifier. Vader now writhed in muffled silence.
Hours went by, and long after Palpatine grew bored with his apprentice's antics he left him there, reawakening the memory with flame when it grew dimmer in Vader's mind. Finally, when Vader was too exhausted to move, even when fire rent an ugly gash across the side of his skull, Palpatine knew he had learned his lesson.
Vader flinched when the lights snapped back on in the room. Palpatine, protected from the heat by the Dark Side, swept in, hypo in hand. Unceremoniously, he jammed the device into Vader's neck, and the man jerked as a heavy narcotic flooded his veins.
"Do you remember who left you to burn, Lord Vader?" he hissed, his anger not yet fully quenched.
Vader's head lolled against his shoulder. His eyes were a disgusting shade of blue again, and totally blank. He had pushed too hard. Palpatine knelt before him, and slapped him hard across the side of his face free of fresh burns.
"Do you remember who left you in the fire, Vader? Do you remember who brings you pain?"
Vader's mouth worked silently, and finally he was able to force a name past his destroyed throat and parched lips.
"K-Ken…ob…i…"
"Good," Palpatine purred. "You do remember. Do you also remember who pulled you out of the fire?"
Vader's eyes lit up. He was able to raise his head slightly.
"You…did…."
"Very good. Do you remember who makes the pain stop?"
"You…do…"
"Do you remember who your master is? To whom you swore loyalty to for all of eternity?"
"Y-you…"
Palpatine grabbed Vader's chin, pulling the man's face close enough that he could smell the scorched flesh.
"Who owns you?" Palpatine underscored his words with a strong Force-suggestion—not that he needed it. Vader's mind was soft. Tears filled his apprentice's eyes—tears of gratitude, of adoration. It flowed through their bond.
"You…do….mas…ter…"
"And do not forget it again, my young apprentice," Palpatine growled. Vader reached out, wrapping his hand around the hem of Palpatine's cloak. The display was entirely too much, and Palpatine booted the man in the jaw, and Vader's grip loosened. He spun, summoning the med droids, who flowed out of panels in the wall. There was a mild amnesiac drug in the hypo cocktail- the details of Vader's punishment would be lost to him.
But he would not forget who his master was again.
