He's not Satan, he just goes down to the corner and gets Satan's cigarettes.
You got it. And when he finds out Luke is his son, his first impulse is to figure out a way of getting him to join him to kill the Emperor. That's what Siths do! He tries it with anybody he thinks might be more powerful, which is what the Emperor was looking for in the first place: somebody who would be more powerful than he was and could help him rule the universe. But Obi-Wan screwed that up by cutting of his arms and legs and burning him up. From then on, he wasn't as strong as the Emperor—he was like Darth Maul or Count Dooku. He wasn't what he was supposed to become. But the son could become that.
--Taken from Rolling Stone #975
And the Emperor was right. Luke had much power in him. It was raw, unchanneled and untrained, but it was vast. His potential was larger than the Emperor's, larger than Vader's.
--Taken from Shadows of the Empire
The Force was powerful; Vader thought the dark side even more so, but he had never been able to use it to heal his badly burned body to the extent that he wished. That he was alive at all was something of a miracle, but he had somehow failed to master the energies needed for complete regeneration. He believed it was possible; that with sufficient meditation and training, he would someday be able to rebuild himself into the man he once had been. Physically, at least.
He would never go back to what he had been mentally. Weak, foolish, idealistic. Anakin had been much like Luke Skywalker was now. Mere...potential. Yes, the Force was strong in Luke, perhaps stronger than it had been in Anakin. But the boy needed to embrace the dark side, to learn where the real power was, to achieve his true promise. If he did not, the Emperor would destroy Luke. Vader did not want that.
--Taken from Shadows of the Empire
Luke paused, for he saw something else, as well; something he hadn’t seen before in the Emperor. Fear.
Luke saw fear in the Emperor—fear of Luke. Fear of Luke’s power, fear that this power could be turned on him—on the Emperor—in the same way Vader had turned it on Obi-Wan Kenobi. Luke saw this fear in the Emperor—and he knew, now, the odds had shifted slightly. He had glimpsed the Emperor’s nakedest self.
Vader watched Luke. His boy was powerful, stronger than he’d imagined. And still pliable. Not lost yet—either to the sickening, weakly side of the Force, that had to beg for everything it received; or to the Emperor, who feared Luke with reason. There was yet time to take Luke for his own—to retake him. To join with him in dark majesty. To rule the galaxy together. It would only take patience and a little wizardry, to show Luke the exquisite satisfactions of the dark way and to pry him from the Emperor’s terrified clutch. Vader knew Luke had seen it too—the Emperor’s fear. He was a clever boy, young Luke. Vader smile grimly to himself. He was his father’s son.
--Taken from Return of the Jedi
A body more powerful in the Force than Vader, potentially more powerful even than Palpatine. The body of a true hero, beloved by all right-thinking citizens in the galaxy as the very symbol of truth and justice...
He would not simply turn Luke Skywalker to the service of the Dark. Why should he? Luke Skywalker served the Dark already, without ever guessing; he had powers of destruction that humbled even the Death Star. No: Cronal would become Luke Skywalker, and serve the Dark himself.
--Taken from Luke Skywalker and the Shadows of Mindor
Luke looked over at us from the small kitchenette and gave us a smile. "Wedge, good to see you again. And you as well, Captain Horn. Can I offer you something to drink?"
"Caf, if you have it." Wedge hid a yawn with the back of his right hand. "You keep it dark enough in here for me to drop off right now."
"Caf it shall be, then." The Jedi Master looked at me and I felt electricity run through his blue-eyed gaze. When we had met before I had felt power in him, but now, after his experiences with the Emperor Reborn, his power had been redoubled.
--Taken from I, Jedi
Brakiss tracked Luke four ways: with the surveillance equipment he had installed all over Telti; with the computer system; with a group of specially designed gladiator droids that silently flanked Luke; and with the Force. His Force sense was the most reliable. Luke's presence felt as if someone had tossed a boulder into the calm pond of Brakiss's world. Although Brakiss had known Luke was coming, he still wasn't prepared for the strength of the disturbance.
--Taken from The New Rebellion
Even Luke's strength in the Force cannot help. The most powerful Jedi Master in the galaxy can only stand by and watch his wife die.
--Taken from The New Jedi Order Sourcebook
By the time of the Battle of Endor, Luke Skywalker has studied some lightsaber technique from Obi-Wan's journal and greatly advanced his abilities. Without a Master, such advancement would be nearly impossible for most, but Skywalker's unparalleled aptitude makes him a match for Darth Vader in their fateful duel onboard the Second Death Star.
But Skywalker's skill at blaster deflection is highly refined, and his lightsaber technique is so superb that he is able to duel the Dark Lord on even footing—and finally able to defeat a man who is a powerful living product of the ancient Jedi sword traditions from time immemorial. Such an achievement with little formal training is a testament to Skywalker's innate abilities and instinctive skill.
--Taken from Insider #62: Fightsaber: Jedi Lightsaber Combat
Luke Skywalker, the most powerful Jedi in the galaxy.
--Taken from Fate of the Jedi: Vortex