Be lion-mettled, proud, and take no care / Who chafes, who frets, or where conspirers are: / Macbeth shall never vanquish'd be, until / Great Birnam wood to high Dunsinane hill / Shall come against him.
Act IV, Scene 1 · The Three Witches
Context
The third apparition, a crowned child holding a tree, tells Macbeth he will never be defeated until Birnam Wood moves to Dunsinane Hill.
Analysis
The prophecy uses an impossibility (a forest walking) as the condition for Macbeth's defeat, which makes his safety sound absolute—yet Shakespeare has the apparition phrase it as 'until,' not 'unless,' subtly keeping the door open. The command 'Be lion-mettled, proud' flatters Macbeth's self-image as brave and regal, ensuring he will not look for the trick in the language; pride becomes the mechanism by which the prophecy traps him.
Essay Tip
Support a thesis that the witches exploit Macbeth's need to see himself as powerful—by framing the prophecy as a compliment ('lion-mettled, proud'), they make him too arrogant to question the impossible condition, so he walks into the trap willingly.