In Act IV Macbeth, feeling his paranoia, returns to the witches after having seen the ghost of Banquo. In Act III, Scene 4, when Macduff fails to attend the banquet, Macbeth becomes very disturbed.
It will have blood, they say: blood will have blood,
Stones have been known to move and trees to speak. (3.4.)
Macbeth questions everyone's motives. Feeling that he has no other recourse, he wants the witches to inform him about what is going to happen. So, on Macbeth's return, the witches summon three apparitions.
1. The first apparition appears shouting Macbeth's name. It is an Armed Head, the symbol for Macduff. It warns Macbeth, "Beware Macduff!" (4.1.74) Macbeth thanks it for the warning and remarks that it has mentioned what he has feared.
2. The second apparition then appears after there is the sound of thunder. This apparition is "a Bloody Child," which is a symbol of Macduff at birth. It calls Macbeth's name three times, then tells him to "Be bloody, bold, and resolute," for then no one born of woman will harm Macbeth. Hearing this, Macbeth becomes confident and says,
Then live, Macduff: what need I fear of thee?
But yet, I'll make assurance double sure,
And take a bond of fate. (4.1.85-86)
So, he decides to kill Macduff anyway and get a guarantee from fate.
3. The third apparition appears; it is a child wearing a crown and holding a tree. This apparition tells Macbeth that he will never be vanquished until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (4.1.97-98)
In other words, the Birnam woods must move toward the castle. Hearing this, Macbeth feels reassured that he will remain king. However, he notes that his heart "Throbs to know one thing." Macbeth still worries whether Banquo's sons will ever reign as kings. He demands the witches tell him. They show him a vision of eight kings and Banquo. Macbeth may not be worried that the third prophecy will ever occur, but he is very worried that the sons of Banquo will reign.