Harry paces around, thinking "it was his fault Sirius had died; it was all his fault" (37.4).
Voldemort took advantage of Harry's "love of playing the hero" (37.4), and Sirius died as a result.
Harry cannot bear to think about this any longer.
A picture behind him suddenly addresses him: it's Phineas Nigellus, Sirius's ancestor.
He asks Harry snidely, "And what brings you here in the early hours of the morning? [...] This office is supposed to be barred to all but the rightful Headmaster. Or has Dumbledore sent you here? Oh, don't tell me [...] Another message for my worthless great-great-grandson?" (37.8).
Harry can't bring himself to tell Phineas Nigellus that Sirius is dead. He can't say the words aloud, or they'll be real.
Dumbledore appears in the fireplace of his office, welcomed by the portraits of the wizards and witches on his wall.
Dumbledore assures Harry that none of his fellow students are going to be permanently damaged by their injuries this evening.
Dumbledore tells Harry, "There's no shame in what you're feeling [...] the fact that you can feel pain like this is your greatest strength" (37.29).
But Harry can't hear what Dumbledore is saying right now – he is in so much pain that he just wants it to stop.
He starts tearing up Dumbledore's office and shouting that he doesn't understand, he can't understand.
Dumbledore informs Harry, "It is my fault that Sirius died [...] Or should I say, almost entirely my fault – I will not be so arrogant as to claim responsibility for the whole. Sirius was a brave, clever and energetic man, and such men are not usually content to sit at home in hiding while they believe others to be in danger. Nevertheless, you should never have believed for an instant that there was any necessity for you to go to the Department of Mysteries tonight. If I had been open with you, Harry, as I should have been, you would have known a long time ago that Voldemort might try and lure you to the Department of Mysteries [...] That blame lies with me, and with me alone" (37.58).
Harry stands stunned, his hand on the doorknob.
He sits back down to hear Dumbledore out.
Phineas Nigellus asks, "Am I to understand [...] that my great-great-grandson – the last of the Blacks – is dead?" (37.62). He can't believe it, and leaves his portrait frame to look for Sirius.
Dumbledore explains that, recently, he has become concerned that Voldemort would become aware of Harry's connection with him through his curse scar.
He has worried that Voldemort would use that connection to possess Harry and provoke a confrontation between Harry and Dumbledore himself.
Dumbledore points out, "On those rare occasions when we had close contact, I thought I saw a shadow of him stir behind your eyes" (37.80).
Harry feels numb as he listens to all of this. Sirius's death makes it all seem so unimportant.
After the attack on Mr. Weasley, Dumbledore was sure that Voldemort would try to possess Harry fully.
So, he told Professor Snape to train Harry in Occlumency.
Through these Occlumency lessons, Professor Snape learned that Harry had been dreaming of the Department of Mysteries for months.
Dumbledore realized that Voldemort would try to use Harry to gain access to his prophecy, making Harry's Occlumency training all the more important.
But, Harry confesses, "I didn't practice, I didn't bother. I could've stopped myself having those dreams, Hermione kept telling me to do it, if I had he'd never have been able to show me where to go, and – Sirius wouldn't – Sirius wouldn't —" (37.87).
But Harry did try to check beforehand, and Kreacher told him that Sirius had gone!
Dumbledore confirms that Kreacher lied.
For months, ever since Sirius told Kreacher to "get out" (23.115), Kreacher used that as an excuse to leave the house. Kreacher has been visiting Bellatrix Lestrange on the sly.
After all, Bellatrix Lestrange was originally Bellatrix Black – the only member of Kreacher's master's family for whom he still has any respect.
So all of this time, Kreacher has been serving two masters.
When Harry came to Number Twelve, Grimmauld Place, Sirius was upstairs tending Buckbeak, whom Kreacher had injured.
Harry is filled with rage at Kreacher, but Dumbledore reminds him, "I warned Sirius when we adopted twelve Grimmauld Place as our Headquarters that Kreacher must be treated with kindness and respect. I also told him that Kreacher could be dangerous to us. I do not think Sirius took me very seriously, or that he ever saw Kreacher as a being with feelings as acute as a human's —" (37.111).
Harry rages at Dumbledore for blaming Sirius for Kreacher's behavior.
He demands to know why Dumbledore isn't angry at Professor Snape. After all, "When [Harry] told him Voldemort had Sirius he just sneered at [Harry] as usual —" (37.116).
Of course Professor Snape couldn't respond openly, since Professor Umbridge was there. Still, he went immediately to the Order to check on Sirius.
Once the Order realized that Harry thought Sirius was in the Ministry of Magic, they mobilized.
Harry is still furious that Professor Snape kept goading Sirius for not joining in the action. Without that provocation, maybe he wouldn't have gone to the Department of Mysteries.
Dumbledore admits, "I trust Severus Snape [...] But I forgot – another old man's mistake – that some wounds run too deep for the healing. I thought Professor Snape could overcome his feelings about your father – I was wrong" (37.124).
Dumbledore also acknowledges, "Sirius was not a cruel man, he was kind to house-elves in general. He had no love for Kreacher, because Kreacher was a living reminder of the home Sirius had hated" (37.128).
Harry then accuses Dumbledore, "You made him stay shut up in that house and he hated it, that's why he wanted to get out last night —" (37.129).
People don't like being locked up, as Harry knows all too well from his own summer experiences with the Dursleys.
Dumbledore buries his face in hands and then levels with Harry.
Five years ago, Harry came to Hogwarts looking thin and unhappy.
Harry had had ten years of neglect and abuse with the Dursleys, so why did Dumbledore make Harry stay there?
Dumbledore wanted to keep Harry alive, so he needed to find a safe place for Harry over the summers.
That place was the Dursley household, where his Aunt Petunia's blood connection to Harry's mother, Lily, protected Harry with ancient magic.
Dumbledore explains, "While you can still call home the place where your mother's blood dwells, there you cannot be touched or harmed by Voldemort. He shed her blood, but it lives on in you and her sister" (37.145).
Harry realizes that it was Dumbledore who sent Aunt Petunia that Howler, "Remember my last" (2.217), the one that made Aunt Petunia keep Harry.
At the end of Harry's first year, when he lay in the hospital wing after his first fight with Voldemort, he asked Dumbledore why Voldemort had come after Harry in the first place.
Even now, Dumbledore wonders if he should have told Harry then about the connection between Voldemort and Harry.
But Dumbledore decided to protect Harry.
And again, in Harry's second year, Dumbledore chose not to tell Harry about their connection.
And again in third year, and even at the end of his fourth year, after Harry watched Cedric Diggory die, Dumbledore decided not to tell Harry the whole truth about his link to Voldemort.
Finally, Dumbledore opens up: "Voldemort tried to kill you when you were a child because of a prophecy made shortly before your birth. He knew the prophecy had been made, though he did not know its full contents. He set out to kill you when you were still a baby, believing he was fulfilling the terms of the prophecy. He discovered, to his cost, that he was mistaken, when the curse intended to kill you backfired" (37.183).
The glass ball in the Department of Mysteries was only a record of the prophecy.
Dumbledore was there when the prophecy was first uttered.
He had gone to the Hog's Head Inn to interview an application for the post of Divination teacher, Sybill Trelawney.
Dumbledore didn't see a trace of the gift in Trelawney, so he thanked her for her time and turned to go.
But then, he heard a strange voice coming from Trelawney.
She said, "The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies ... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not ... and either must die at the hand of the other for neither can live while the other survives ... the one with the power to vanquish the Dark Lord will be born as the seventh month dies" (37.191).
There were two boys born in July of that year whose parents had defied Voldemort three times: Harry and Neville Longbottom.
But it is Voldemort who made the choice about which boy would fill the prophecy.
Voldemort marked Harry with his curse scar, leaving them forever connected – Harry, a half-Muggle just like Voldemort himself.
The one good thing about this prophecy business is that the guy who told Voldemort about the prophecy had only been able to overhear the beginning, about the boy being born in July to parents who had thrice defied him.
Voldemort doesn't know that Harry "would have power the Dark Lord knows not —" (37.210).
Harry protests that he hasn't got any special or secret powers.
Dumbledore tells Harry that's not true: Harry's heart is what saved him tonight from Voldemort's possession.
Harry asks, so, does this prophecy mean that one of us has to kill the other?
Dumbledore says yes.
He then continues hesitantly, "I feel I owe you another explanation, Harry [...] You may, perhaps, have wondered why I never chose you as a prefect? I must confess ... that I rather thought ... you had enough responsibility to be going on with" (37.218).