In Act III of "Macbeth," the title character comes to realize he is trapped by the witches' prophecy in what he calls a "fruitless reign." Driven by a paranoid desire to secure his crown, which was gained through murder, Macbeth plots yet more killing and dispatches murderers to slay his friend Banquo.
The witches' prophecy foretold that Macbeth would sit upon the throne of Scotland, but that the crown would eventually pass to Banquo's family. In Act III, Macbeth tries to short circuit this destiny by having Banquo and his son murdered in the night. The attack is only partly successful, with the contracted murderers ambushing and killing Banquo, but losing their torch and letting Fleance escape. The survival of Banquo's son seals the Scottish king's destiny and dooms him later in the play.
While plotting multiple murders, Macbeth experiences a crisis of conscience. He confesses to his wife, who had earlier driven him on to murder Duncan, that his mind was "full of scorpions" as he faced what he had become. The events of Act III combine to foreclose any hope that Macbeth could escape the prophecy of dying without an heir and being replaced by the family of his victim.