The instruments of darkness tell us truths

• Banquo warns that the witches — “instruments of darkness” — use half-truths to lead humans to destruction.
• The metaphor “instruments of darkness” frames the witches as tools of a larger evil, likely the devil. 
• Banquo’s insight might show his moral strength — unlike Macbeth, he recognises danger in seductive supernatural power, which is why he resists.
• His warnings heighten the dramatic irony — the audience sees Macbeth walk into destruction, despite Banquo’s clarity.