The King has gone. From the wall Silva takes down two swords, releases his guest from his hiding place, and bids him cross swords with him to the death. Ernani refuses. His host has just protected his life at the danger of his own. But, if Silva insists upon vengeance, let grandee and bandit first unite against the King, with whom the honour of Elvira is unsafe. Elvira rescued, Ernani will give himself up to Silva, to whom, handing him his hunting horn, he avows himself ready to die, whenever a blast upon it shall be sounded from the lip of the implacable grandee. Silva, who has been in entire ignorance of the King's passion for Elvira, grants the reprieve, and summons his men to horse.
He sets on foot a conspiracy against the King. A meeting of the conspirators is held in the Cathedral of Aix-la-Chapelle, in the vault, within which stands the tomb of Charlemagne. Here it is resolved to murder the King. A ballot decides who shall do the deed. Ernani's name is drawn.
The King, however, has received information of the time and place of this meeting. From the tomb he has been an unobserved witness of the meeting and purpose of the conspirators. Booming of cannon outside tells him of his choice as head of the Holy Roman Empire. Emerging from the tomb, he shows himself to the awed conspirators, who imagine they see Charlemagne issuing forth to combat them. At the same moment the doors open. The electors of the Empire enter to pay homage to Charles V.
"The herd to the dungeon, the nobles to the headsman," he commands.
Ernani advances, discovers himself as John of Aragon, and claims the right to die with the nobles—"to fall, covered, before the King." But upon Elvira's fervent plea, the King, now also Emperor, commences his reign with an act of grace. He pardons the conspirators, restores to Ernani his titles and estates, and unites him with Elvira.
Silva, thwarted in his desire to marry Elvira, waits until Ernani and Elvira, after their nuptials, are upon the terrace of Ernani's castle in Aragon. At their most blissful moment he sounds the fatal horn. Ernani, too chivalrous to evade his promise, stabs himself in the presence of the grim avenger and of Elvira who falls prostrate upon his lifeless body.
In the opera, this plot develops as follows: Act I opens in the camp of the bandits in the mountains of Aragon. In the distance is seen the Moorish castle of Silva. The time is near sunset. Of Ernani's followers, some are eating and drinking, or are at play, while others are arranging their weapons. They sing, "Allegri, beviamo" (Haste! Clink we our glasses).
Ernani sings Elvira's praise in the air, "Come rugiada al cespite" (Balmier than dew to drooping bud).
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