Hi! I'm so sorry that I've taken quite a while to write this review, but life always, always gets in the way. :P
I must add that I would have reviewed this sooner, but I wanted to read it twice just to make sure I was talking about the right things. Before I go any further, I will say that your prose is as good as ever, but this is a very tricky thing to write about, especially since so much fanfiction is so fluffy and unserious. You tackle the deep, dark, scary stuff, and I think you've done a brilliant job thus far. :)
The section about silence and words was wonderful. I could definitely see how the themes ran through the rest of the chapter, and you captured it all so brilliantly. The first person voice can become so dull, but you kept me interested with the content and the tone in which this was written. I can picture Dumbledore saying the words, as if he was telling a story to Grindelwald's broken body after their great showdown in the 1940s.
I did spot a typo at the very end, but it might be a stylistic thing: "For he and I, life was a single, long conversation from the moment we met." In this sentence, the "For he and I" doesn't really jive for me. If you take out "he," it reads "For I, life was...". You need to replace "I" with "me," and perhaps "he" with "him." So the sentence would be "For him and me...". Trust me, it's grammatically sound, although it might not seem so. :)
One overall suggestion that I'm going to offer you is that you watch your commas. I personally like the way your prose breathes--the commas are like inhales and exhales, which is particularly beautiful. But it sometimes became a little too much, and occasionally I had a bit of trouble following the main idea of the sentence.
Ooh, but that lazy August morning scene is beautiful. You've got me shipping Albus/Gellert already, and that's mean of you because I know how this ends (in sadness). I love the imagery of the birds outside the window and the continental summer, and how maybe the scene is tainted with the rosy retrospective with which we look back on the past. Albus really loved Gellert, didn't he?
I like how, although the scene mostly sticks to itself, the few mentions of the future really color the action. What "papers" are telling of Gellert's new flames? Newspapers? Did he take companions and dump them on his rise to power?
Oh dear. "We never talked much in the mornings; the silence was lovely then, but now I cannot help but wonder if there was only silence because neither of us knew what to say, or if it was because we knew everything there was to be said." This is so lovely and so true (just like everything else you write). It establishes the uncertainty of youthful passion and the fallible certainty of youthful arrogance. Forever isn't real for them, but had they spoken more, they might have had a little longer.
"It all started with a crescendo." Brilliant musical imagery there. I pictured an orchestra, poised to begin, and at the first note, they do something of a sforzando before the real tension begins. (Sorry, musical terms are what I live for when I'm being a music major.) It was rather brilliant of you, and it set the tone for the nasty, raging minor key riffs that were to come.
"...Aberforth, my spell missing him by mere inches, attacked me in return, believing on your side." Did you mean to put "believing me to be on your side" there? I felt like there were a few words missing.
YES. More musical imagery. Lovely, and so, so terrible. I imagined a crash of cymbals just before the silence. Did you listen to an orchestral work as you wrote this?
You captured Albus's feelings perfectly. It was a bit melodramatic, but he acknowledged his melodrama and apologized for what he was about to do. I don't think he sounded too whiny at all. It's all a part of reminiscing about the past--you have to look back on it and see all of the mistakes and the regrets, and it can all pile up on one's soul. Whatever he's about to do, his memories must have some relevance. I'm interested to see what will happen. Is he on his way to fight Gellert? That will certainly be an interesting scene.
Brilliant beginning! Hopefully the next review I give you won't be so very tardy. :D