After learning of his family's fate, what does Macduff vow to do?
Macduff has just learned that Macbeth has had his wife and all of his children murdered. He has to, he tells Macolm, "feel it as a man", and he then makes the following vow:
But, gentle heavens,
Cut short all intermission; front to front
Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!
Macduff ask the heavens to cut out any gap or interval of time between NOW and the time he gets to see Macbeth (the "fiend of Scotland") face to face. If he escapes Macduff's sword, Macduff says, let heaven forgive him his sins. But, as we find out in the play's penultimate scene, Macduff's sword is not for Macbeth to escape: it kills him and cuts off his head.
Hope it helps!