I did. But the demonic character at the end I introduced only very late.
I was asked to do a silk-screen painting and then I though of that character:
a large image of him above New York with Rork somewhere in the distance below.
I never finished the drawing, but I saved it, and used it in
Le cimetière de cathédrales
because I knew then how the series would finish.
I like that in a series: you create a universe, in which accordingly
you can do whatever you like. You can reuse some elements,
add other things, things that occured before the story began, etc.
I have to take of a truely immens thing, and I actually like that very much.