Hecate, the "mistress of the witches," doesn't show up until Act III, scene 5. Like Macbeth, we as an audience have believed the three Weird sisters were frightening and powerful—until Hecate's arrival.
Hecate is very angry that the witches have acted without consulting with her first. She is also angry that they have helped Macbeth. She doesn't think he's worthy of their aid, and she tells them she knows he won't do anything for them in return for their help. She calls him "spiteful and wrathful" and out only for himself. Therefore, she wants the witches to trick him and lead him to his doom.
While all along we have been frightened of the Weird Sisters, we see them cowed and intimidated by Hecate. She scolds them severely saying:
Saucy and overbold, how did you dare
To trade and traffic with Macbeth
In riddles and affairs of death,
And I, the mistress of your charms,
The close contriver of all harms,
Was never called to bear my part,
Or show the glory of our art?
She gives the witches orders to meet her later and says she will conjure "artificial sprites" that will confuse and confound Macbeth. The witches are relieved when she is gone and want to get away fast before she comes back. They say:
let’s make haste; she’ll soon be back again.
Clearly, Macbeth has been dealing with amateurs. Now that Hecate is on the scene, he is in real trouble, although he does not know it.