Ma Pei has been unimpressed by Du Ge's observation that they have arrived at the ghost beach, 'an ancient battlefield' where 'ghosts appear at night and... tell the secret thoughts of your mind'. When the witches appear to him, however, in a musical gust of 'wind and hail', Ma Pei is all too impressed.
The witches prophesy Ma Pei's imminent promotion to the position of 'King next to the Imperial Throne' ('yizi binjian wang' , a regal title originating in the Tang dynasty and used as a loose equivalent of 'Thane of Cawdor') [still#2].
The warning 'against General Du' has no counterpart in the equivalent scene in Shakespeare, but is taken from Macbeth's second meeting with the witches. Its appearance at this point in the adaptation is part of a general tendency to make Ma Pei seem less culpable (more tempted) than Macbeth. It anticipates Tie Shi's (Lady Macbeth's) stratagem of 'warning' her husband against the emperor.