This letter would be dedicated to Sebastian at the end of season one, after Sebastian has swallowed Ciels soul. Ciel has left the letter behind and ensured that Sebastian would receive it after his death thus the content of the second season is not included.

Dear Sebastian.

I took me a long time to think about the beginning of this letter. "Dear friend" would be a lie, but "Dear Butler" could not fully describe what had been between us in those years. However. When you read this, then I am dead with a high possibility. Probably you have… swallowed my soul as you always wanted, if not then I am truly sorry you really well-deserve it.
You were always there, caring for me in a way no one other would be simply able to and I know I never showed real gratitude to you. I thought it would be written. I took everything as given and never knew how to treasure the opportunities you gave me.
It must have been hard to you all the time baking pies, cooking curry, and buttoning my shirts and to bow in front of me; though I understand that things like money or nobility have no value to you. I was never able to show my feelings since that day… You tried to teach me how to smile, unfortunately with less success but the try made me happy for a while.
You deserve something better than my torn soul; it has become little more than a shell of vindictiveness and useless pride. I can feel now that our journey will come to an end soon enough and when I was at the beginning more then determent and sure of this way, but an angel wing gave me doubts and they fell on fertile ground.
I knew you always wanted me to be strong. I was always aware of the things you did to strengthen and shape my soul and me. I know that you knew that I have known that and you always asked yourself why I did let it happen, although you never asked.
You gave me a feeling of security and care.
I could never tell you that face to face, cause my biggest fear were never to die or loose the game I played, but to disappoint you.
The reason why I am still alive is to kill my parents' murderers in a slow and cruel way, as they did to them. Hatred and pain became part of me like my name and blue eyes, but it is true that I have had a live and that's all owed to your gentle service.
I had a dream once before all that began. It was about being invincible, powerful and mighty, that is what every Phantomhive is supposed to be. Fact is that I could never have reached all this without you. I've come so far with you and I never thanked you. I would have to led my guard down and that was not capable for the men I ought to be.
Nonetheless. In the end it doesn't even matter.

Thank you.

Nothing will remain and remember of me once I'm gone right? All the revenge I want so badly will be like a fragile flame in a light ocean of fires, to less to notice it. It doesn't even matter if I reach my goal. No one will recognize, no one but you and me.
The time with you was never boring that I can tell you for sure. Indeed I had a live worth living.
Let me tell you something. I may not know how it feels being a demon, but I guess I learned to understand you as a person a bit. I know what was happening outside when those gangsters attacked the manor. I recognized all the little things you did in secret for me. Maybe that surprises you, maybe not, but based on that I allow myself to believe that you do not see me just as me meal but a little more than that and so you are maybe even a bit… sad about our ending?
If that should be true, then I order you to stop that.
I order you to not be unhappy. You did your best to hide it but even you have a soul and a heart and while we were together you've changed.
First you were cold, unsympathetic and somehow constrained and you may not want to believe me or, your demon rules forbids you to do so, but I think you may could have even felt something similar to attachment to me, besides your craving after my soul. Maybe not... I was never able to really comprehend you.
All I know is that you are not the full evil like you always pretend to be. All our proud we have had was useless in the end. In the end I've put my trust in you and you did not let me fall, that is the only thing that does matter.
I tried so hard to win the game but even if I would win, there will be nothing left but you.

Sincerely, faithfully, yours, Ciel Phantomhive

Sebastian stared a long time on the letter with this familiar, clean handwriting and a hint of melancholy crossed his face.
His hands ran over the paper, following the phrases, then he folded it, and put it in his pocket. Wasn't there a slight smile as he went down along the river against the moon?