One of the main themes of William Shakespeare's Macbeth is how opportunities to gain wealth and power test the moral nature of characters and cause them to change towards either good or evil. In the play, the precipitating factor is supernatural. Three witches or "weird sisters" appear on a blasted heath in the opening scene, and give the ominous warning that "Fair is foul, and foul is fair" (Macbeth I.i).
Macbeth and his friend Banquo encounter the witches after a victory in battle in which they appear loyal followers of King Duncan and noble and valiant warriors. The three witches predict that Macbeth will become Thane of Cawdor and subsequently king and Banquo a father of kings. Once the witches' predictions of Macbeth's new title prove true, the two characters begin to change. Macbeth becomes more ambitious and less scrupulous, beginning to speculate on the possibility of becoming king, while Banquo worries that no good can come of listening to evil witches, stating: "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..." (Macbeth I.iii). Over the course of the play, Macbeth becomes increasingly evil in his pursuit of absolute power, murdering King Duncan in his sleep, and eventually arranging to have Banquo killed, while Banquo becomes increasingly aware of moral and religious duties, stating "In the great hand of God I stand; and thence/Against the undivulged pretence I fight" (Macbeth II.iii).
The third character who changes over the course of the play is Lady Macbeth, who makes a decision to help her husband achieve his ambitions. She prays (to evil spirits) for the strength to be strong and unscrupulous enough to strengthen Macbeth in his goals: "Come, you spirits/ That tend on mortal thoughts, unsex me here,/ And fill me from the crown to the toe top-full/Of direst cruelty! make thick my blood;/Stop up the access and passage to remorse ..." Although she does in fact strengthen Macbeth's resolve and acts as a force for evil in the play, remorse and guilt drive her insane by the end of the play.