I stayed up to read this far too late last night, then went to bed and had a lovely dream about Lucius, so your story definitely wormed its way in there!
I really, really liked it: clever world-enhancing details, an intriguing and suspenseful plot and structure, characterisation so vivid I half expected them to turn around and snarl at this voyeur to go away. Though on second thoughts being watched wasn't something that bothered these three.
I'm so glad you included the scene between Arthur and Lucius - that Arthur, while hating Lucius, could nevertheless see him as a fellow human being in his loss. I can't help feeling that Lucius would have acted very differently had the positions been reversed.
The other character moment that stood out was Regulus meeting Severus for the first time. I've not seen anything about them at that age before, and I loved the idea of Sirius' badmouthing Snape leading to Regulus admiring him. How galling Sirius would find that!
I also liked Lucius' observation of the freedom of having nothing left to lose. A new (and surprising) sensation for him, I suspect! This, and your sparsely effective description of his grief, and a hundred other details, build up an emotional truth that made the story ever more real.
I am a little confused about Narcissa's death - is it significant, who killed her? I wasn't sure if I missed something there. I would also have liked to know more exactly about what Lucius (or rather Severus) was planning to achieve with the essentia? Not that it ultimately mattered to the story, but you piqued my curiosity.
I was sorry it ended the way it did, sorry Lucius didn't manage to do what he was striving to do. That's not a criticism, but an indication of how much I cared. It's a rare pleasure to read a deeply imagined Lucius in a story in which the smut doesn't smother the substance. This one's definitely going on my favourites list.