Cover and background texture: it’s quite pretty. The cover art is evocative and a little mysterious.
The worn paperback texture works well on the outer cover, though I find the same worn look on the inside pages a bit jarring. If this was genuine wear, the inside would be less worn than the outside. The folded corner on the inside middle panel does not seem possible (and there is no corresponding fold on the outside panel).
Layout: Very effective use of fonts and colour. The map is beautiful. I like the way the formidable warrior artwork uses the same palette as the map and text. Overall layout is very clear, everything is understandable at a glance.
Prelude: I didn’t get it on my first reading. After reading the rest of the intro material, it made more sense on a second reading. The relationship of the local people to the Pale Prince is a bit ambiguous. He is the “elven master”, so like a fief lord? Or do the elves keep to themselves and the local people being affected by his malaise is purely circumstantial? Maybe you could add a sentence to explain the relationship between the people and the elves, it seems like important context.
The adventure: I love the coherent theme of the complex: a series of tombs of dead lovers, each with their own unique characteristics.
Presumably the locale is within Crescent Hill? Or is it a structure atop Crescent Hill? The intro only mentions, “under which the paramours of the Pale Prince rest” so I assume it’s underneath. What is the entrance like? An archway on the hillside? A hole in the middle of a ring of faerie stones? Is there a door?
System neutral is fine, but unique monsters could perhaps use some explanation. What is a “ditsy skeleton”?
It is not clear to me how the spirit of Lorelei can be interacted with besides reading the poem to her. Does she respond if spoken to? If she is attacked do weapons just pass through her, does she fight back, or does she just disappear?
Was Angoirix also one of the prince’s lovers, or is the prince’s sexuality left to the discretion of the GM?
The wandering monsters fit with the theme but it seems like a lot of creatures wandering around quite a small complex. I suppose some of them could be encountered outside the tomb.
Overall: it’s a nice adventure with strong theme, good use of environmental storytelling, and mix of combat and puzzle challenges. Would require a little work by the GM to make decisions about the ambiguities above and create stats.