"All of your life now you have denied
There'd be a time when you'd ever die
Still it's been rumored this thing must be

Why is it then that you act surprised
When I appear now to be your guide?
Why do you hesitate to follow me?

See it rising, stare and wonder
Hear it beckon you to dance
Feel it hold you, take you under
I'm your God of second-chance."

'Mephistopheles' - Trans-Siberian Orchestra

Carreau is Cillian's inner demon - technically that means that they are joined closer than the closest lovers.
Cillian is a religious one. The rosary is for when he needs to keep Carreau at bay for a while, so that he can concentrate on work, or anything else important.

Carreau [left], Cillian & Image © redderz