The ravaged city of Aquileia. Attila, “the scourge of God,” has invaded Italy. The Huns celebrate their victory as a group of captured female warriors is brought in. Their leader, Odabella, is the daughter of the Lord of Aquileia, whom Attila has killed. Odabella declares that Italian women will always defend their country. Impressed by her courage, Attila offers to grant her a favor and Odabella asks for a sword. When Attila gives her his own, she vows to use it against him to avenge her father. After the women have left, Attila summons the Roman general Ezio, greeting him with respect as a worthy adversary. Ezio proposes a secret bargain: the entire Roman Empire shall be Attila’s if he makes Ezio ruler of Italy (Duet: “Tardo per gli anni”). Attila denounces Ezio as a traitor and angrily rejects his offer.
The shores of the Adriatic. A storm rages across a mudflat in the Adriatic lagoons. Once it has passed, the noble Foresto arrives with a group of Aquileian refugees. He is worried about Odabella, his fiancée (“Ella in poter del barbaro”). The refugees greet the sun as a sign of hope and Foresto urges them to build a new city there between sea and sky—the future Venice.
Attila’s camp, outside of Rome. Odabella has stayed with Attila as his army approached Rome, hoping to find an opportunity to kill him. Looking at the night sky, she imagines seeing the faces of her father and of Foresto, whom she also believes dead (“Oh! nel fuggente nuvolo”). Suddenly Foresto appears. She is overjoyed but he, having seen her with Attila, accuses her of betrayal. Odabella convinces him that all she wants is revenge, like the Biblical Judith, and the lovers are reconciled.
Attila’s tent, at night. Attila recounts a terrifying dream: an old man confronted him at the gates of Rome and, in the name of God, denied him access (“Mentre gonfiarsi l’anima”). Recovering his composure, he calls on his troops to march on the city, as a procession approaches. It is led by the Roman bishop Leo—the old man of Attila’s dream, who now repeats the same words. Attila is horrified as the Christians praise the power of God.
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Ezio’s camp. Alone, Ezio muses on Rome’s fallen state (“Dagl’immortali vertici”). Foresto, now Attila’s slave, invites Ezio to a banquet and suggests a plan for a surprise attack on Attila during the feast. Ezio is excited at the thought of avenging his country.
Attila’s banquet. The King of the Huns welcomes Ezio and the Romans to his camp. Foresto tells Odabella that Attila’s wine has been poisoned. Feeling cheated out of her revenge, she warns Attila of the poison as he is about to drink. Furious, Attila demands to know who is responsible. When Foresto steps forward, Odabella asks that the right to punish him be given to her in return for saving Attila (Ensemble: “Lo spirto de’ monti”). Attila agrees and declares that as a sign of gratitude, he will marry Odabella the next day.
A Wood. Foresto awaits news of Odabella’s marriage, lamenting her apparent treachery (“Che non avrebbe il misero”). Ezio arrives and tells Foresto that his men are ready to attack the Huns at his signal. As the wedding procession is heard in the distance, Odabella suddenly appears, distracted and begging her father’s ghost for forgiveness for marrying the man who killed him. When Foresto confronts her, she protests that she always loved him. Attila enters, searching for his bride; he finds Odabella with Foresto and Ezio and accuses all of them of disloyalty and ingratitude. All three answer with hatred (Quartet: “Tu, rea donna”). As distant cries signal the beginning of the Roman attack on the unsuspecting Huns, Odabella stabs Attila with the sword he gave her, embraces Foresto, and cries out that they are avenged.