Ermione returns, extremely agitated. She does not know whether she loves or hates Pirro, and regrets ordering Oreste to kill him (Aria: "Parmi, che a ogn'istante de' suoi rimorsi al grido" / "I imagine that at any time he may utter a cry of remorse). Oreste appears, wild-eyed, stumbling and holding out the bloodstained dagger. He tells Ermione that she is avenged, (Duet: "Sei vendicata" / "You are avenged") and describes how Pirro spared Astianatte and swore to make him his heir. Surrounded by angry soldiers drawing their swords, Pirro was attacked and killed, but not by Oreste, who says that he had given the dagger to another man and then reclaimed it. Ermione accuses Oreste of murder, and he realises that she was still in love with Pirro. She asks the Eumenides to destroy Oreste.