And on thy blade and dudgeon gouts of blood,
Which was not so before. There’s no such thing:
It is the bloody business which informs
Thus to mine eyes.
– William Shakespeare
Macbeth, Act 2, Scene 1. When Macbeth hallucinates the dagger just before murdering Duncan, he sees blood on the blade and handle. He realizes that the dagger is not real, but is a sign of his guilt over plotting to kill Duncan. The blood also foreshadows Duncan’s murder.