I read this novel or novelisation one afternoon at the city library somewhere between late 1984 and early 1986.
It was set in the Star Wars universe and the main story was that there was some sort of crystal on a planet that could amplify /boost the power of a force-user/adept.
Luke and Leia are both on the planet where this crystal is and are trying to stop the Empire, specifically Darth Vader, getting hold of it.
I think they find a small splintered-off piece of the crystal early on and find that it works as advertised and are then terrified of what the whole thing would be able to do.
At one point Luke is incapacitated or they get separated (not sure which) and Leia faces Vader alone. She has this total BAMF moment where she fights Vader with a lightsaber and drops him off a cliff.
I don't remember the ending or much else of it, although I think the crystal was either destroyed or buried beyond any force user's ability to retrieve.
Sorry I don't have more details, but I was only 13 or 14, and speed read it in under two hours while waiting for a friend to finish at the dentist, and it seems like another lifetime.