My dear old friend:
You once said that the face you wore every day, the face that now smiles at us every morning from the newspaper, was your true mask, but it was not long before you betrayed those words yet again. It has hurt me sorely, losing you, but I am patient--ever so patient, as I have had to be--and I believe things can now be set right between us.
I can be forgiving, as you know. Perhaps it is time to extend the hand of friendship to you again. Or perhaps I should be harsh, and send the hammer of war, instead. The choice, old friend, is purely yours.
Your eldest son has recently come into my company. He came here of his own free will, but I am afraid I have not yet been convinced of his true motives. This may take time, and further investigation on my part and that of my dear, trusted associates. It need not be so. He is little more than a child, yet, and I am sure you would protect him from this battle we engage in. To have him returned, I would ask only a few simple tokens of your sincere desire to resume our previous relationship:
You must arrive at a location to which I will guide you, this Friday evening no later than 11pm. You will bring with you the spells and passwords necessary to gain unlimited entry to the Ministry of Magic, including the Department of Mysteries. And you will take a new Oath, and swear to me that you will not stray again. You will endeavour to make my victory a quick one, this time, that we may move on from all of this unpleasantness.
Then you will help me with a small matter that has been giving me considerably inconvenience for quite some time. But of this, we will talk more later.
Do not fail me.
[there is no signature, but the sender is unmistakeable, if for no other reason than the way the Mark burns when in close proximity to the letter.]