The witches are sometimes seen as representing fate, or destiny. do they merely reveal what will happen, or do they manipulate events?

In Macbeth (1623) by William Shakespeare, the witches are seen as representing destiny but they only manipulate events.

They do not predict the future, they only make assumptions based on their knowledge about Macbeth's love for power. Furthermore, the witches, who have great power in the play, use their ability to persuade in order to convince Macbeth and Banquo that the prophecies are real. Therefore, the witches end up being also responsible for Macbeth's actions and crimes. In the play, Macbeth does not control his own destiny, the witches do. Macbeth kills Banquo because they "predict" that his friend will be the father of a line of kings. In that way, the witches not only manipulate Macbeth's fate but they also represent his darkest side.  

a fable" i don’t have much to go on

do you have more to the question?