The history of this wallpaper dates 2 years back. I took a screenshot of the 2nd season's ED and made a vector (this one), but had no clue about the background, so it just laid there for a year. Then, I found this scan. Suddenly I had so many ideas about this wall, to make Ciel a demon, to get a big couch for him and make a fairly simple background.
I used this scan for the couch. Its design gave me the idea for the hanging cage, as for the skull, it's Supernatural's merit. In Supernatural demons are clouds of smoke that possess people, they don't have a body of their own. The only way to kill them is by burning their human bones. So the skull in the cage is Ciel's. Demon Ciel has a body, but it's separated from his human one. He just carries his own remains around.
Initially in the cage was a raven, symbolizing Sebastian's eternal contract, but I liked the skull idea more. The outfit is an idea of mine, I just modified his original clothes and added lace and ribbons. Special thanks to Twilightanimal for her help with the horns.
Here's a textless version.