Acacis stabs himself when he realises that Montezuma will win Orazia and furthermore aggravated about his own future prospects as the son of his mother, an unlawful usurper.
The old Ynca is moved by all these events and offers Orazia to Montezuma - before Montezuma receives the information that he is actually the heir of the Mexican throne, the very person who can with all justification marry the Peruvian princess.