Sounds kinda hokey. I am a fan of Veitch though. Lucas doesn't read SW novels as much as comics, since he's a visual guy. I wonder what aspect of Dark Empire drew him in, the unique art or the out-there plot.The comic is now something like 25 years old, but I'll put it in spoilers anyway. This doesn't cover the whole plot, just the general idea of Luke's fall and redemption from the Dark Side
Six years after the Battle of Endor, Luke gets captured. He discovers the Emperor is alive. Luke attempts to defeat the Emperor using his skills as a Jedi. Fails. The Emperor has found a way beyond death, so nothing Luke does will defeat him. Palpatine again suggests that Luke join him to replace the long dead Vader, as is his destiny. Luke agrees because he sees no other way He cannot defeat the Emperor unless he discovers how he is surviving death. He uses this time to attempt to undermine the Empire's advances and learn the Emperor's secrets. But he falls more and more to the Dark Side...it clouds his thinking. Only Leia can manage to pull Luke towards the Light enough for Luke to challenge the Emperor a second time after learning how the Emperor is cheating death. He again loses (losing his mechanical arm again). He now is more like Vader...serving in fear of the Emperor ,while trying to find a way to overthrow him. Again Leia comes. She manages to draw Luke back to the Light after he attempts one last time to fight with the Dark Side, that fails, but with both Skywalkers fighting Palpatine in the Light....they get the Emperor really mad. So made that he calls up a Force Storm to destroy all the Rebels on the planet below. But the Skywalkers have learned something. They combine their efforts in the Light and ever so briefly distract Palpatine from his fury. Cut him off from the Dark Side for a moment. Just long enough for him to lose control of the Storm, which turns back on him. (This is not the end of the story, but that's the arc of Luke's fall and redemption. The rest of the comic and the few novels that reference the events have Luke understand both the Light Side and the Dark Side and become a generally better Jedi Master for it. He still loses some students to the Dark Side, but many of them he manages to bring back because of his personal experience in the matter.