In what way is the dagger similar to the prophecies?

The bloody dagger, in Act II, scene i of Macbeth, foreshadows Macbeth's murder of Duncan. Initially, the bloody dagger leading to Macbeth is "showing" Macbeth what must be done ... the dagger, then, represents the means by which he can succeed. Second, the dagger is an apparition, and Macbeth realizes it.