"Legend": Pretty, but did anyone ACTUALLY understand the plot?

Okay, so I finally caved and watched Ridley Scott's "Legend" (the theatrical cut, not the director's cut, which I know is different). Visually? Stunning. Seriously, the set design, costumes, and practical effects are top-notch. Tim Curry as Darkness? Iconic. No complaints there. Mia Sara's Lili was also super ethereal and beautiful. But... the story? I'm genuinely confused. Was anyone actually clear on what was going on? Like, why does Darkness want to plunge the world into eternal night beyond just being, well, evil? And what was the specific deal with the unicorns? Was it just killing them that unleashed the ice age, or was there more to it? The quest felt so... vague. In the book (if there WAS one, which I can't seem to find evidence of??), I feel like a lot of the plot should've been fleshed out more. Specifically, the scene where Lili is corrupted by Darkness. It's visually striking, but the transition felt clunky. Did she consciously choose to embrace the darkness, or was it more like she was tricked or possessed? Maybe the longer cut makes it make more sense? Did anyone else feel like the plot was secondary to the visuals, or am I totally missing something here?