In Shakespeare's Macbeth, the first apparition in Act Four, scene one, is an armed head. This apparition (image) that appears out of the pot in which the witches have woven their spell represents Macduff.
The first apparition delivers the following prediction:
Macbeth! Macbeth! Macbeth! Beware
Beware the Thane of Fife...(78-80)
Macbeth is thankful for the warning, which confirms his suspicions of Macduff.
Whate'er thou art, for thy good caution, thanks;
Thou hast harp'd my fear aright. (81-82)
He already has concerns about the Thane of Fife because he did not attend the banquet the night Macbeth had Banquo murdered. However, the next apparition tells Macbeth that no one can defeat him that was born of a woman. Since literally this cannot be true, Macbeth believes Macduff is really not a worry, but will order his assassination and that of his family—just in case.