Sam had no idea how soon. If Dean had been taken to Hell in his dreams, then soon, Sam would dream of his return.
And the start of the wheels set in motion that would bring about the end. In this still-human-yet-not-quite form, there were moments where Castiel struggled intensely with the way his dream being blindly followed the commands of Zachariah and Raphael. So unquestioning.
Such a soldier.
"I have never played darts, Sam." Would that things always remained so simple. Without a doubt, Sam would soon dream of his weaknesses after Dean's death.
And how could things fail to be complicated after such Revelations?