What mood is established at the beggining of the play?

From the very beginning the mood is dark and "creepy."  The first scene, with the witches, establishes a bit of contrast:  "fair is foul and foul is fair."  The witches plan to meet Macbeth on the "heath" where the environment would be foggy and basically sort of overcast.  Even the battles of the civil war cast a pall on the whole story.