When Jacen felt Luke undergoing a form of great torture through the Force, the first thing he did was power up the Jade Shadow's engines. However, mere seconds after that, he received a transmission; he had a feeling that it was probably coming from spaceport security telling him that he hadn't received proper clearance for departure from his docking port.
But he didn't care, not when his uncle's life was on the line, so he ignored the transmission; and he didn't even think about any police forces that would be on his tail even after he lifted off. Jacen nearly collided with two incoming ships, but he managed to swerve out of the way each time before he soared off for the coordinates where Mara died.
Once Jacen was over those coordinates, he set the Shadow to hover in place twenty feet over the street below before he stood up and headed for the boarding ramp, which had already been lowered for him to disembark. He then leaped out and landed in a Force-cushioned crouch on the road that didn't seem to have any traffic and headed for an alleyway that had a manhole cover already lifted up and out of the way.
But before Jacen could even enter the alley, Luke suddenly leaped into view from out of the open manhole and landed at the alley's threshold.
Jacen was both shocked and scared by the death glare that his uncle was giving him and the sense of unmitigated, passionate rage that emanated from his Force-presence.
Before Jacen had a chance to ask him what was wrong, Luke had his lightsaber off his belt and activated. Not even a second later, the Jed Grand Master leaped across the gap between himself and his nephew to try to strike Jacen down.
But Jacen instinctively brought up his own saber to block the strike, yet he now found himself being backed across the street toward the wall of the building behind him as he dueled defensively against Luke.
"Uncle Luke, what are you doing?!" Jacen managed to ask just before his back hit the wall.
He then leaped up and over his uncle to land right behind him, only to be kicked in the back; Jacen landed prone on the road, but managed to turn to his sore back just in time to block Luke's next strike. He then kicked Luke's left kneecap, bringing him into a kneeling position, before just as promptly kicking up at his uncle's jaw; Jacen used that momentum to bring himself back over into an over-the-shoulder roll that returned him to his feet.
However, Jacen was then blown back off his feet by a powerful Force-shove from Luke that landed him in the alley from which the Grand Master emerged. And right before Jacen even landed, Luke was in the air so that he could soar down to deliver a killing stroke upon his nephew.
Jacen was ready for him, though; even as he was still on his back, Jacen provided a Force-shove of his own that knocked Luke out of his intended descending arc and now had him falling back to the street. Then, before Jacen could give his uncle time to recover, he leaped back to his feet and gave yet another Force-shove that knocked Luke back to the wall behind him.
The Grand Master's back crashed against the wall behind him and he fell prone to the sidewalk that was across the street from Jacen, who now left the alley to see if his uncle was all right. But Luke swiftly looked up, his face contorted more in anger than in pain, and he raised his free hand—his natural one—up to deliver a torrent of Electric Judgement, a technique that Jacen remembered seeing his uncle use during the final battle of the Yuuzhan Vong War.
Jacen caught the torrent on his lightsaber, but now he found himself being forced to stay in place as he used his own Force-powers to redirect the flow of electricity into the neutralizing agent that was his lightsaber.
And that effort only increased for Jacen not only with each passing second, but with each step that Luke now took toward him.
"Uncle Luke, stop this!" Jacen called out. "Please! Why are you doing this?! What's wrong with you?!"
Luke didn't answer; he only continued to close the distance to Jacen with each subsequent step.
Finally, the power of the Electric Judgement became so great that it blew Jacen so far back into the alley behind him that his back hit the wall there; as his lightsaber clattered away from his grip, he collapsed to the ground.
But Jacen only lay there for two seconds before he was raised bodily into the air by Luke, who was now gripping him with the Force by the throat. Naturally, Jacen began to struggle as his breath and vision began to fade along with his life.
But even through this losing battle, Jacen saw Luke enter the alley and say, "Now you will pay for what you've done."
"Uncle Luke!" Jacen creaked out through hoarse breath. "This isn't you! You don't... do this! You're... the Grand Master... of the... Jedi... Order! Not... your father... Darth... Vader!"
But Luke's rage continued to permeate through his Force-presence as well as in his outward expression. However, his hold did lighten around Jacen's throat as he pulled his nephew closer to him.
"Funny you should compare me to Vader," Luke growled, "when, with her dying words, Mara compared you to Palpatine."
Jacen was confused by that statement; but he didn't have time to think about it before he was tossed back by another Force-shove from his uncle. Once again, his back hit the wall behind him, but this time, he was pinned there, and the invisible grip on his throat tightened again.
As he felt his lungs beginning to collapse, Jacen couldn't help but look down at his lightsaber nearby. And even though it was getting really difficult to concentrate, he managed to return his gaze to Luke before he Force-nudged the weapon over to the Grand Master's feet; the older man didn't seem to notice, as his rage seemed to distract him that much from the danger now being posed to him.
Then, with just the tiniest bit of the Force, Jacen brought his lightsaber up by a few inches even as it was simultaneously activated.
The blade pierced through Luke's right leg just below the knee.
The Grand Master's concentration broke as he collapsed to his back in pain. Jacen fell to his knees, coughing furiously before inhaling lungfuls of precious air, even as he winced from the pain that his back took during the fight. He then stood up and walked over to his downed and wailing uncle, where he grabbed the lightsaber hilt and deactivated the blade.
Jacen then stomped Luke right in the face, knocking him unconscious before he could give the wounded man a chance to use any of his Force abilities against him.