In Act IV, Scene I, the witches show Macbeth the following three things:

1. An "armed" head (wearing a battle helmet), which tells Macbeth to beware of Macduff.
2. A "bloody child" who tells Macbeth that he should be fearless because "no man of woman born" can ever harm him.
3. A child with a crown on his head and a tree in his hand appears next. He tells Macbeth to be brave and proud like a lion because he will never be defeated until Great Birnam Wood to Dunsinane Hill marches to fight against him.

Macbeth then asks the witches a specific question about Banquo's sons. The witches tell Macbeth to not ask any more questions, but he will not be appeased until they have given him an answer. This prompts the witches to show Macbeth the fourth (and final) apparition.

Macbeth then sees a line of eight kings marching past him, of which the last of these holds a mirror in his hand. At the end of the line is the ghost of Banquo.

This is all the evidence Macbeth needs. He now realizes that Banquo's sons will indeed be kings.