It is highly recommended you read the new Darth Vader comic series that came out in 2017. This is going to be a loose recap and continuation of the events that occur in issue #5. I do not own Star Wars…Disney does ;)
Prologue: Return to Mustafar
His fight with Master Infil'a was ultimately successful though it came at a price. The skilled ex-Jedi Master had thrown him from the mountain top like a child's plaything. He has lost one of his leg prosthetics and damaged an eyepiece of his helmet to the point it no longer function. The prosthetic had been replaced with a droids leg. In fact he had used several parts from that droid to repair most of his suit. It would have to do temporarily.
A heavy price but worth it, he thought as he sat back in the captain's chair of his personal transport. His thoughts drifted to a relaxing soak in the Bacta tank followed by a long, mask-free, session in his meditation chamber. Inevitably his com-link soon chimed, flipping a switch it appeared he had received a message prior to returning to his ship.
Answering it revealed Darth Vader's master, Darth Sidious, the feared Emperor Palpatine. The evil, smug grin upon his gnarled face angered Vader, despite it being a hologram. Like any Sith he served his Master faithfully but desired to overthrow him and rule in his place.
"Lord Vader. If you are seeing this message, then your quest is almost at an end. The droid flying your ship was instructed to take complete control of your ship upon sensing you obtaining a Jedi lightsaber. You'll soon arrive at your final destination…a destination of my choosing," the Emperor said with a sinister cackle.
Vader looked towards the front viewport and could see a large volcanic world approaching rapidly. Vader anger towards his Master swelled as he recognized where he was being taken.
Mustafar..
"Mustafar is unique Lord Vader. Deep within the planet there is a place where the Dark Side is enhanced. Return the place of your greatest failure and reclaim yourself! Return to me more powerful than ever!"
Vader's ship had cleared the atmosphere; it was daylight on the planet. Though the many volcanoes spewed dark smoke into the air, it was still a sunny day. Much different than the night Vader was last here he thought vengefully…the night when he was defeated and left for dead.
"You may have bested that simple minded fool but his saber and its crystal will be even more of a test! You cannot simply pick up his weapon and call it yours. It is alive and full of the Light Side of the Force. Search for the location where the Dark Side is strongest on Mustafar…feed off of it…use its power. Use your strength to bend the saber's crystal to your will"
Reaching out in the Force it did not take long for Darth Vader to sense a massive well of Dark energy. With a simple thought, he guided his ship in the direction of the energy and soon saw the opening of a cave.
"There is much hate and anger within you, my apprentice. Make them your tools and make the crystal bend to your will. Force it to accept the Dark Side. Corrupt it. Make it BLEED!" the emperor said with delight as the hologram blinked off and the landing dock opened.
The air was still as hot and oppressive as he remembered. His walk was slowed due to the droid leg and he struggled to walk over the rough terrain. The Dark energy seemed to draw him in, lending its power and rejuvenating
His functioning eye piece adjusted to the gloom within the cave and Vader found the temperature to be nearly unbearable in the cave. His burnt, scarred skin began to stick to the insides of his suit as he began to sweat. It itched. Some parts of his skin couldn't sweat anymore, making the suit that much more uncomfortable. He suppressed a surge of annoyance, deciding to use it for breaking the crystal.
Walking to a nearby natural formation shaped like a large, round table, he took Master Infil'a's saber out and dismantled it. The emerald green crystal was exposed and it pulsed with energy, casting a dim glow across Vader's helmet. Grasping it between his index and thumb he examined it closely. This would be much different than when he first built his lightsaber. He did not have to make any of his kyber crystals bend to his will before. It seemed to so small…so fragile. He knew better than to underestimate the crystal…after all they provided the power for a lightsaber.
Placing the crystal in the very center of the table, Vader felt out for it through the Force. The crystal was surrounded by a thick wall of energy. Reaching a dark gloved hand out, he pushed a fraction of his hate and anger onto the crystal.
It resisted.
Strongly.
Vader poured more of his strength into the crystal.
A loud hum emanated from it as it began to shudder violently.
"Nnnnf…" Vader strained, managing to lift the crystal off the table through the Force. It shown brightly as it fought back, bathing the entire cave in a brilliant green light. At this point Vader was pouring every ounce of his rage into the crystal and yet it stood strong against his might.
SZZZZCK!
"GAH!" With a loud crackle and a flash of light Vader was launched backwards into the cave wall, bringing chunks of rocks down upon him. The damaged eyepiece shattered as one of the rocks struck it. His head swam as he struggled to stay conscious as another wave of white hot energy rolled over him. And just like that, his head cleared.
"What…" he said sitting up.
"…What have I done?"