In this important soliloquy, we are presented with Macbeth, who is waiting for the signal from his wife to go and kill Duncan. He imagines a dagger which he cannot grasp, which leads him to contemplate what he is about to do and helps him prepare himself:
Thou marshall'st me the way that I was goin;
And such an instrument I was to use--
The dagger therefore can be seen to be moving towards Duncan's room, richly suggestive of the violence that Macbeth is just about to engage in. In sharp contrast with Banquo, Macbeth allies himself with the forces of witchcraft and evil:
Now o'er the one half-world
Nature seems dead, and wicked dreams abuse
The curtain'd sleep; Witchcraft celebrates
Pale Hacate's off'rings; and wither'd Murther,
Alarum'd by his sentinel, the wolf,
Whose holw's his watch, thus with his stealthy pace,
With Tarquin's ravishing strides, towards his design
Moves like a ghost.
Note how the "summons to sleep" that Banquo feels has actually become a "summons to hell" for Duncan according to Macbeth in the last line of this powerful soliloquy. Thus in this soliloquy we are presented with a Macbeth whose subconscious creates a dagger that he sees covered in blood, that moves towards the chamber where Duncan is sleeping. Macbeth uses this dagger to discuss his perceptions of reality, and then, based on these musings, firmly allies himself with the forces of evil just as Lady Macbeth did in Act I scene 5. The scene is set for the horrendous crime of regicide that is to follow.