Unity of the play Macbethis achieved by the use of?

Really, any of these things could affect the unity. The images of darkness and blood tie the story together, from the beginning to the end. The comic relief is seen in the porter scene and some in the scenes with the witches when they are doing their witch "thing." (Double, double toil and trouble, etc.) Certainly the unity is achieved through Macbeth being brave at the end and almost facing his death bravely, dying better than he lived (as is described in the Thane of Cawdor early in the play).