In Shakespeare's Macbeth, Act Three, scene one, Macbeth delivers a soliloquy with regard to being King and the dangers that threaten his position, namely his close friend Banquo.
Macbeth states that to be King is nothing if he cannot safely be so. His biggest worry at this point is Banquo, who was present when the witches made their predictions. This incident would naturally raise questions in Banquo's mind about what Macbeth did to make the predictions come true. Macbeth fears Banquo's "royal nature," in essence, his integrity. Banquo has the courage to take a stand and is smart enough to do so in safety. Banquo, Macbeth admits, is the only person he fears, and Macbeth's actions would be "despised" in Banquo's sight—the same way Caesar felt about Mark Antony (or so they say).
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’Tis much he dares, (55)
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valor
To act in safety. There is none but he
Whose being I do fear; and under him
My genius is rebuked, as it is said (60)
Mark Antony's was by Caesar.
Macbeth recalls that Banquo scolded the three witches ("sisters") when the first "predicted" that Macbeth would be king, that they should speak to him also. Like prophets, the witches declared that while Banquo would not be king, he would father a line of kings. Macbeth understands now that he will never have children to pass his crown to ("fruitless crown") and his power will be taken from him ("barren scepter"), never to be given to his son: he has no children.
He chid the sisters,
When first they put the name of King upon me,
And bade them speak to him; then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown(65)
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding.
If this is the case, Macbeth reasons, he has sold his soul to the powers of darkness—in killing Duncan, a king—so he could be King, but more so, that Banquo's descendants will come to the throne. Seemingly for nothing, Macbeth has lost his peace of mind, has lost his soul ("his eternal jewel") to "man's common enemy" (the Devil), just to make Banquo's "issue" (descendants) kings. Rather than see this happen, Macbeth is prepared to fight Fate to the death if necessary, to keep the witches' prediction for Banquo from coming true.
If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered, (70)
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!
Rather than so, come, Fate, into the list, (75)
And champion me to the utterance!