A threat of punishment for disobedience. The words are from Virgil's AEneid (i. 135), and were uttered by Neptune to the disobedient and rebellious winds.
“Neptune had but to appear and utter a quos ego for these wind-bags to collapse, and become the most subservient of salaried public servants.” —Truth, January, 1886.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Quos Ego from Infoplease: