The beginning was so slow and over-descriptive that I was very tempted to hit the next button. But I'm less quick to do that these days because I know I'll have to wait another week for a new episode, so I kept listening, and I'm glad I did!
I'm not very familiar with any of the authors that others have said this evoked. I've only heard one Lovecraft, and that was The Music of Erich Zann here on PP (which I very much enjoyed). So I'm coming into it with fresh eyes.
Overall, I wish the first half had gotten to the point a little faster. Lots of description, not much happening. Lots of writers do this (Tolkien being the master of over-describing) but it's not my cuppa. And, as others have pointed out, the "indescribable" and "unspeakable" adjectives constantly making an appearance weakens the otherwise powerful descriptions.
It really started interesting me when each member of the harem was brought out, and it kept me interested until the end. And by "end" I mean the point where the "hero" leaves. I prefer to retcon out the Bond-villain explanation at the end and leave it at least marginally ambiguous.
To me, the most powerful things in this story were what I saw as the two themes. Yes, these themes have been covered well elsewhere, but I thought this was a decent addition.
1. The all-powerful quest bends the hero to its will. Similar to Memento in this way. The quest becomes the sole focus of the hero's attention for so long that it gains a life of its own. No longer is the quest a means to an end, but it is both means and end. "The Quest Must Go On" is similar "The Show Must Go On" as if the quest/show are more important than anything, but without any explanation for WHY. The Hero has scraped away every bit of his soul that could interfere with his Quest, and when he finally attains his goal, he finds that he still feels empty inside, unfulfilled, and if he stops the Quest he will have nothing to do but wallow in his own misery. Continuing on the Quest doesn't make him feel any more fulfilled, but it gives him a purpose and he can push aside his despair in his efforts to drive on.
A Quest-obsessed hero can be one of the most capable villains because some part of him feels that what he is doing is for Justice and Truth, and he doesn't give a crap that he just tortured hundreds of innocent priests to death.
I'm not sure it would've ended any happier if he had taken Fiona back with him. I'm not convinced he was any more sane or any less cruel than the evil priest, and with no quest to follow, he would've turned his sadistic attentions on Fiona.
2. Reality can't compare with nostalgia. He remembers Fiona through the distorted filter of his own nostalgia, and over the years, his obsession with the quest to find her has only strengthened the distortions in this memory. The version of Fiona in his mind is so flawlessthat no flesh-and-blood woman could possibly compare. When faced with the real incarnation, even one which has been apparently put in stasis to prevent the inevitable aging that SHOULD have occurred to turn her into a wrinkled old lady, all he can see are the flaws not represented in his memory. Even kept so young, the real Fiona is human, and therefore she cannot be perfect enough. He sees her in this moment of weakness coming out of the chamber and her fragility, her mortality is so apparent that he knows in a moment that she cannot possibly be the immortal, incomparable, unchanging, inhuman being he has used to drive himself to continue on his quest. She has to eat and poop like any other person, she has flaws and tempers like any other person, she gets sick sometimes and she will die like any other person. No matter how strong he is in other ways he cannot cope with the realization that he has gone to such lengths and done such terrible things for a woman who is only human, so instead he denies the truth and abandons her.