After the battle in which Macbeth defeats the traitorous Macdonwald, he and Banquo happen upon the three "weird [meaning "Destiny-serving"] sisters who make the predictions that Macbeth will be Thane of Cawdor and "King thereafter!" and Banquo, who will not be king himself, will beget sons who will themselves be kings--"Lesser than Macbeth, and greater"(1.3.65). After hearing these predictions, Macbeth demands,
Say from whenceYou owe this strange intelligence, or whyUpon this blasted heath you stop our wayWith such prophetic greeting. Speak, I charge you. (1.76-79)
shall be kings,
When those that gave the Thane of Cawdor to me
Promised no less to them? (1.3. 122-124)
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths.
Win us with honest trifles, to betray's
In deepest consequence (1.3.132-135)
This supernatural solicitingCannot be ill, cannot be good. If ill,Why hath it given me earnest of success,
Commencing in a truth?
Are less than horrible imaginings
...and nothing is
But what is not. (1.3.138-145)
Of direst cruelty! Make thick my blood,
Stop up th' access and passage to remorse
That no compunctious visiting of nature
Shake my fell purpose....(1.5.33-37)
The role of the three witches is to generate imagery, mood and atmosphere. One has to remember that in Shakespeare's days, the existence and power of twitches was widely believed. Therefore, they are the first major influence on macbeth's action.
The opening scene of the play closes with the rhyming couplet "Fair is foul, and foul is fair, Hoev through the fog and filthy air." Through this contradiction in terms the witches reveal that they are about to invert the values as we know them. This is so because they say that what is good is bad and what is bad is now good.
Before meeting them macbeth was content at Thane of Glamis until they hail him also as "Thane of Cawdor and....king hereafter."I.3.48,49) Macbeth is stunned by these prophecies. That information does tempt macbeth. He orders them to "stay you imperfect speakers. Tell me more..." He is keen to hear more of this "strange intelligence... speak I charge you." (I.3.68-76). However for fairness sake, it must be said that the witches never invited Macbeth to murder Duncan or even suggested such a thing. The weird sisters were just there to symbolise evil, but man is free to resist them. macbeth is tragic because he comes to depend upon their information. "....would they have stayed" (I.3.80)
The witches also speak the strange prophecies to Banquo. He describes the physical appearece of the witches. He comments on their clothing and says that they cannot possibly be human beings. "So wither'd and so wild in their attire, That look inot like the inhabitants o'th'earth". This shows that Banquo questions what he sees and that he likes to analyse what goes on around him.
When Lady macbeth reads the letter that Macbeth sent her, in which he informed her about the three prophecies and his new title of Thane of Cawdor, she instantly starts thinking about what can be done in order for her husbance to attain what he hwas promised - Kingship. in this very important soliloquy, she expresses her fear that Macbeth is too good natured and kind hearted to be able to do what it takes to attain the crown, this is kill Duncan. She therefore callss upon the evil spirits "....Come you spiritis that tend on mortal thoughts, unsex me here. And fill me from the crown to toe topfull of direst cruelty". She is literally calling on demonic spirits to possess her mind and soul so as to get rid of all human pity. After the murder is committed Lady Macbeth's evil character disintegrates into insanity