Read it through once

Act third was the castle hall; and here Hagar appeared, having come to free the lovers and finish Hugo. She hears him coming, and hides; sees him put the potions into two cups of wine, and bid the timid little servant "Bear them to the captives in their cells, and tell them I shall come anon." The servant takes Hugo aside to tell him something, and Hagar changes the cups for two others which are harmless. Ferdinando, the "minion," carries them away, and Hagar puts back the cup which holds the poison meant for Roderigo. Hugo, getting thirsty after a long warble, drinks it, loses his wits, and, after a good deal of clutching and stamping, falls flat and dies; while Hagar informs him what she has done in a song of exquisite power and melody.