How did the witches set the tone of the play in Macbeth?

Interestingly, Shakespeare introduces the audience to the witches before introducing Macbeth himself (or any of the other characters for that matter). They enter the scene accompanied by thunder and lightning, forbidding omens in their own right, and they describe a battle raging, a "hurly-burly" after which they will meet with Macbeth. The audience can guess from their presence and their words that they are up to no good, though there is no indication of exactly...

Interestingly, Shakespeare introduces the audience to the witches before introducing Macbeth himself (or any of the other characters for that matter). They enter the scene accompanied by thunder and lightning, forbidding omens in their own right, and they describe a battle raging, a "hurly-burly" after which they will meet with Macbeth. The audience can guess from their presence and their words that they are up to no good, though there is no indication of exactly what they will attempt. As the scene ends, they depart, singing that "fair is foul" and "foul is fair." This song portends some of the foul deeds that Macbeth will undertake in order to secure and maintain his throne. Again, the audience is not exactly clear about what will happen, but they are alerted, as it were, that evil deeds may be afoot, that all may not be as it appears, and that the witches themselves may play a role in the proceedings.