Disclaimer: I do not own Kuroshitsuji, nor do I own the fairytale The Shadow by Hans Christian Andersen, on which this one-shot is based.
In the beginning, Sebastian had no face.
To tormented Ciel he appeared as a vague silhouette; a distorted, disproportioned black smudge with bright shapeless eyes, as thin as a veil of smoke and as insubstantial as the end of a nightmare as it clings to the waking world.
When the contract was formed he acquired painted features, delicately brush-stroked and a little too animated; a little too perfect to be quite real. All he had to work with was an almost monochromatic pallet. White face, black hair, black clothing, black contract on the back of a white hand. But bright eyes like drops of blood made him more colorful than any human in London.
"You shall be my master, and I shall serve only you," he told Ciel, with lowered blood-drop eyes, the statement as simple and truthful and clear as black and white.
Ciel knew only that Sebastian's face reminded him of someone, and did not suspect that the fact was of any importance.
In time, Sebastian's face changed. It grew both sharper and more blurred; it lost its agelessness and became more human. He looked, Ciel began to realize, a bit – just a bit – like Ciel's father.
It was a very odd coincidence, as if the artist who inked his features had used Ciel's father as a model.
But this did not last long. As the contract aged and Ciel with it, Sebastian grew ever…
His every expression grew softer and his eyes grew wider, and at first Ciel was proud that he had grown so much that he nearly matched Sebastian in height.
And then he looked straight into Sebastian's eyes one day and realized that they were his own. Blood-drop red but unmistakable. Sebastian did not look like Ciel's father anymore.
For years Sebastian trailed Ciel as if he were his shadow, serving him as their contract required; changing, growing, shrinking, shifting, and never leaving his side. For years, Ciel was the master, and Sebastian served only him.
And revenge, when it finally came, was all well and good.
But Ciel was tired.
The world, for him, had become distorted, smudged, and shapeless. He felt as thin as a veil of smoke and as insubstantial as a daydream in a world of nightmares.
A mere shadow of his former self.
So Sebastian took his soul and ate it, and when he was finished he said, "I shall be your master, and you shall serve only me."
The shadow on the ground at Sebastian's feet had no face, and did not answer.
Sebastian's blood-drop eyes turned blue, and when people called him by the name Ciel, he did not correct them.
He took Ciel's title and married Ciel's fiancée, and lived happily with wealth and status.
His shadow was only a shadow.
It changed and grew and shrank in the light, as shadows always do.
In the dark, it disappeared completely.