First, can I just say what a beautiful, beautiful, elegant, dark and desperatly sad story this is, this has really caught me up inside, and i shan't be forgetting it in a hurry.
Along with the wonderful prose, what really makes this fic is your characterisation of Regulus. His thoughts are so detailed, get so deeply into his mind and emotions. For example, I particularly liked his fear of death that goes right back to his childhood, and the way this is contrasted with Sirius's bravery makes the effect so much stronger. I especially loved the line 'I do not understand how to die.' Ooooh that sent shivers down my spine. The fear shows a vunerable side to him, almost cowardly, however his actions at destroying the horcrux contradict that, adding another side to him.
The dialogue we see from his childhood seems to be reflected in his dialogue even as an adult, his hope of not being killed, his stargazing, and this all reflects the vunerability of his position.
You've made me feel as though I was almost inside Regulus's mind. I'll definatly be reading more of your writing!