RALPH: As we mentioned earlier, witches were believed to have made a contract with the Devil, and to have been given evil spirits as assistants. That certainly sounds like what’s going on here with Lady Macbeth’s mention of ‘murdering ministers’. 

DAVINA: It was also believed that witches breastfed their evil spirits. But instead of providing milk, the witch’s breasts secreted bile, the bitter liquid stored in the gall bladder.

RALPH: We also learn that the "murdering ministers" are in their "sightless substances”; that is, they are in their natural form, which is invisible to humans. They can only be seen when they assume the form of an animal, such as a rat or a cat. 

DAVINA: Finally, the evil spirits are “waiting on nature’s mischief”; that is, waiting to cause some natural disaster like a storm.

RALPH: But “nature’s mischief” could have a second meaning. These agents of the devil are waiting for a failure in someone’s nature – Lady Macbeth, for example – so that they can convert that person into an instrument of darkness.