One of the main event in the story is when Macbeth and Banquo discover the prophesy of the three witches that says that Macbeth will be named Thane of Cawdor, and King after, while Banquo's heirs shall be Kings.
This is an important moment because this is what will make Macbeth and his wife decide to kill their monarch to take his place. This is also when Macbeth became ambitious and when he learns that Banquo's heirs will be king after him and not his children.
When the two generals of King Duncan encounter the weird sisters, they were just comming back from a battle against a rebellion. They were going throught a vast plain when the witches appeared and disapeared.
After this event, Banquo and Macbeth became more suspitious between each other, which will lead to Banquo's murder.
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