What is the result of Macbeth's second visit to the three witches?
On the second visit to the witches, Macbeth receives a series of prophecies that will be of great significance at the end of the play. The audience, importantly, has already learned that Hecate plans to make a series of apparitions appear that will make Macbeth overconfident. So when Macbeth is confronted by three apparitions, they know that he is choosing to interpret their predictions in an overly-optimistic light. The first apparition, an armored head, says to beware of Macduff. This seals the fate of Macduff's wife and child, who are murdered when Macbeth sends assassins to Macduff's palace. The second is a bloody child (as in a newborn) that tells Macbeth that he cannot be harmed by anyone born of a woman. This, of course, comes to pass when it is revealed that Macduff was born by caeserian section, not a normal birth. But Macbeth doesn't consider this possibility at the time. Third, he is informed by a child holding a tree that "Macbeth shall never vanquished be until Great Birnam Wood to high Dunsinane Hill shall come against him." This comes to pass when the troops assaulting Dunsinane cut boughs from the trees in Birnam Wood as they march on the castle. Finally, the witches conjure a procession of kings, the last of which holds a mirror in which can be seen a reflection of Banquo. In this scene, as in others, Shakespeare cleverly raises questions, without resolving them, about the role of the supernatural and that of human agency.
Macbeth visits the witches for the second time in act 4, scene 1. Macbeth wants to know about the future, so the witches cast a spell, causing some apparitions to appear. Speaking directly to Macbeth, the apparitions make a number of prophecies.
Firstly, Macbeth is told that he must be wary of Macduff. Secondly, he is told that no man "of woman born" can harm him. Thirdly, he is told that he cannot be "vanquished" until Birnam Wood comes to Dunsinane.
As a result of these supernatural messages, Macbeth becomes confident that he cannot be overthrown by his enemies. In addition, he swears that he will act on the "firstlings of his heart," meaning that he will do whatever comes to mind without thinking about it. This relates specifically to Macduff because Macbeth sends his henchmen to kill Lady Macduff and her child. For the reader, this is a clear sign of Macbeth's growing tyranny and thirst for blood.