At one fell swoop means a swift, decisive blow. WS probably dreamed up this phrase that he used in MacBeth. Perhaps he was thinking of a hawk or falcon coming out of the sky and striking its prey. That would seem to be the context in the play.
In the play we see MacDuff exclaiming on hearing that his family and servants have all been killed.
"All my pretty ones?
Did you say all? O hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop?"
The kite was a bird of prey common in Tudor times in England.
