This Interlude more or less sets up the ending of the book.
Interlude: A Last Warning
April 10th, 1994
Dear Mr. Potter:
It has been some time since I have written to you, I know. I have been busy collecting information, at great personal risk to myself. The journey was not so bad. The destination most assuredly was, and if anyone had guessed what I was doing, the consequences would have been incredibly harsh.
I have prompted you to ask why Sirius Black did not go to Azkaban. However, you have not been diligent in seeking the answers. That will not do, Mr. Potter. You must begin to take your duties seriously—and that includes your duties as a leader of importance to both Light and Dark wizards as well as your duties in protecting your brother.
As you assuredly did not know, Sirius Black was your parents' original Secret-Keeper. But then his brother Regulus was captured when attempting to leave the Death Eaters, in possession of an object that Voldemort valued very much. Alas, I have been unable to learn what this object was.
Voldemort used a curse to connect Sirius's and Regulus's minds, and let Sirius feel his brother's pain as he tortured him to death. In so doing, he intended to break Black and make him give away your parents' location. But Dumbledore intervened before he could. Peter Pettigrew became your parents' Secret-Keeper in Black's place, with instructions to betray you at an appropriate moment. If that does not convince you of Dumbledore's malice and Pettigrew's innocence, then nothing ever will. Make a less wary connection with Pettigrew, I urge you. I believe he has come to seek you from pure motives.
This information I learned from Pensieves. There is one more piece of information, which I learned from a tapestry, and which may be of interest to you:
Sirius was disinherited by his parents when he was sixteen, after he ran away to your father's house. That resulted in his mother blasting him off the Black family tapestry and making his brother Regulus heir. However, the tapestry I have seen has Sirius's name printed clearly on it.
I believe this is the reason that Regulus's existence was concealed from you, your brother, and so many other people, as well as the circumstances of his death. There are spells that will work against inheritance magic if the designated heir is both dead and forgotten by enough people, and if the person selected as heir instead is related to the family by blood or adoption. It is the only reason I can give for why Sirius Black's name is still on the tapestry as the heir of his line, rather than the inheritance having passed to his cousin, Bellatrix Black Lestrange.
For some reason, Sirius wanted to remain his parents' heir, and Dumbledore agreed.
Think on that, Mr. Potter, and be careful whom you trust. This is one of the last times that I will be able to help you pass one of the harsh tests of life. Soon, you must take your place as leader, and then there will be no one to shield you. You must make your decisions, and those decisions will include killing.
Yours in starlight and shadows,
Starborn.
April 10th, 1994
Lucius:
You will note already that you do not recognize my handwriting. That does not matter. Be assured that I hold the same position as the last person who wrote to you. However, he has proven…most unsatisfactory as a servant. He has been replaced.
You do not know me by my writing, Lucius. This quill was guided by unfamiliar hands. However, I know you. I can imagine your eyes widening with shock, then narrowing as you try to process what this will mean for you and your family. Have no fear, my sly and cunning Lord of Bad Faith. I will tell you what it means.
It means destruction. You and your family were doomed from the moment you chose to ally yourselves with Harry Potter. I have been patient long enough. There is no forgiveness now. And my true allies, now escaped from the isle of mists and unhappy memories that obscured them for so long, will not forgive you, either.
You will have heard about the attack of spiders on your best hope for the future? That was only a demonstration of my power, Lucius. Be assured, if I wanted him dead, he would be. However, I do not yet want Harry Potter dead. I have far more specific uses for him in mind. There will be justice before I am finished.
The Dark Lord has almost returned, Lucius.
Now comes the night, and out of this darkness, there will be no morn.