Well, this isn't criticism, only praise, but please don't skip it.
This was an amazing story, extremely well-written with a timeless style. This is exactly how I imagined Regulus; a brave and smart student who was influenced too easily. I love the introduction to the possibility of Regulus in Ravenclaw.
The only little bit of criticism that I have falls under the useless category, as it is only my preference and opinion. I've always imagined Mrs. Black hating Gryffindors because she was a Slytherin, with the house rivalry and whatnot, and forever disapproving of Sirius' friends. I never really thought about Mr. Black, but I think you created his character extremely well.
Overall, one of my favorite fanfics because of the JKR style of making the magical world more than unicorns and fairy princesses; making it into something concrete and believable.
I think the only thing that could possibly make this better is another letter, like Draco's dying wish after the war and when his son is grown up.

Author's Response: Firstly, I'm sorry i've taken so long to respond to this, because this is a perfect example of the kind of review that I absolutely adore. Secondly, thank you, so so much, I honestly never thought it was that good, but I'm glad you like it. Thirdly, the reason I wrote that Mrs. Black liked her sons was because the amount of hatred JKR gave the woman in the books didn't seem like enough to actually make it likely, much less possible for Sirius to have lived to the age of fifteen. I did try my best and again, I'm glad you enjoyed it, again, sorry I took so long to respond. xx