The Oresteia is a trilogy of tragedies about the end of the curse on the House of Atreus, written by Aeschylus.

It is the only surviving trilogy of ancient Greek plays, although the fourth satyr play that would have been performed with it has not survived. The plays were originally performed at the Dionysia festival in Athens in 458 BC, where they won first prize.

Agamemnon

Agamemnon details the return of Agamemnon, King of Argos, from the Trojan War to his death. Waiting at home for him is his wife, Clytaemnestra, who has been planning his death as revenge for the sacrifice of their daughter, Iphigeneia. Furthermore, in the ten years of Agamemnon's abscence, Clytaemnestra has entered an adulterous relationship with Aegisthus, Agamemnon's cousin and the scion of a dispossessed branch of the family, who is determined to regain the throne he believes should rightfully belong to him.

The play opens to Clytaemnestra awaiting the return of her husband, having been told that the mountaintop beacons have given the sign that Troy had fallen. However, when Agamemnon arrives, he has in tow as a slave and concubine, the prophetess Cassandra. This, of course, serves to anger Clytemnestra further.

The main action of the play is the agon between Clytaemnestra and Agamemnon. She attempts to persuade Agamemnon to step on a purple (sometimes red) tapestry or carpet to walk into the their home. The problem is that this would be indicative of hubris on Agamemnon's part, and he does not wish to do this. Eventually, (the reasons why are highly debated) Clytaemnestra does convince Agamemnon to enter the house, where she kills him in the bath: she casts a snare on him and as he struggles to free himself she hacks him with two strokes of an axe.

Whilst Clytemnestra and Agamemnon are offstage, Cassandra discusses with the chorus whether or not she ought to enter the palace, knowing that she too will be murdered. Cassandra is a daughter of King Priam of Troy. She has been cursed by Apollo to possess the gift of prophecy, but at the same time that her prophecies would not be believed by those who heard them, although they were true. In Cassandra's speech, she runs through many gruesome images of the history of the House of Atreus, and eventually chooses to enter the house knowing that she cannot avoid her fate.

The Libation Bearers

The Libation Bearers is the second play of the Oresteia. It deals with the reunion of Agamemnon's children, Electra and Orestes and their revenge.

In the palace of Argos, Queen Clytaemnestra, who now shares her bed and the throne with her lover Aegisthus, is woken up by a nightmare: she dreamt that she gave birth to a snake, and the snakes feeds from her bosom and draws blood instead of milk. Alarmed by this, a possible sign of the gods' wrath, she orders her daughter the princess Electra, whom in the meantime Clytaemnestra has reduced to the virtual status of a slavegirl, to pour libations on Agamemnons' grave. A group of women (the Libation Bearers of the title) are to assist her.

Electra arrives at the grave of her father and comes upon a man by the tombstone, who is placing a lock of his hair on the stone. As they start to speak, it gradually and rather agonizingly becomes apparent that the man is her brother Orestes (who had being sent away to the royal court of Phocis since infancy for safety reasons), and who has, in her thoughts, been her only hope of revenge. Together they plan to avenge their father by killing their mother Clytaemnestra and her new husband, Aegisthus.

Orestes wavers about killing his own mother, but is guided by Apollo and his close friend Pylades, the son of the king of Phocis, that it is the correct course of action. Orestes and Pylades pretend to be ordinary travellers from Phocis, and ask for hospitality at hte palace. They even tell the Queen that Orestes is dead. Delighted by the news, Clytaemnestra sends a servant to summon Aegisthus. Orestes kills his mother first, and then the usurper. As soon as he exits the palace, the Furies appear and, being only visible to him, they begin to haunt and torture him for his crime. He flees in agony.

The Eumenides

The Eumenides is the final play of the Oresteia, in which Orestes and the Furies go before a jury of Athenians, the "Areios Pagos" (Rock of Mars, a flat rocky hill by the Athenian Agora where the supreme criminal court of Athens held its sessions), to decide whether Orestes' murder of his mother, Clytaemnestra makes him worthy of the torment they have inflicted upon him.

Orestes is tormented by the Furies, chthonic deities that avenge crimes of blood. He, at the instigation of his sister Electra, has killed their mother Clytaemnestra, who had in her turn killed their father, King Agamemnon, upon return from Troy. Orestes finds refuge at the temple of Apollo in Delphi, and the god, unable to deliver him from the Furies' wrath, sends him to Athens under the protection of Hermes, while he casts a spell of sleep upon the pursuing Furies in order to delay them.

Clytaemnestra's ghost appears and rouses the sleeping Furies, urging them to continue hunting Orestes. The Furies' first appearance on stage is haunting: they hum in unison as they wake up, and seek to find the scent of blood that will lead them to Orestes' tracks. Ancient tradition says that on the play's premiere this struck so much fear and anguish in the audience, that a pregnant woman miscarried on the spot.

The Furies' tracking down of Orestes in Athens is equally haunting: Orestes has clasped Athena's statue in supplication, and the Furies close in on him by smelling the blood of his slain mother in the air. Once they do see him, they can also see rivulets of blood soaking the earth beneath his footsteps.

As they surround him, Athena intervenes and brings in a jury of twelve Athenians to judge her supplicant. Apollo acts as attorney for Orestes, while the Furies act as spokespersons for the dead Clytemnaestra. In the end, we wind up with a hung jury and Athena breaks the tie by voting in favour of Orestes, and then must persuade the Furies to accept her decision. They eventually submit, and Athena renames them "Eumenides" (Ladies of Good Will). Athena also declares that henceforth hung juries should result in the defendant being acquitted, as mercy should always take precedence over harshness.

That the play ends on a happy note may surprise modern readers, to whom the word tragedy denotes a drama ending in misfortune. The word did not carry this meaning in ancient Athens, and many of the extant Greek tragedies end happily.

Worth noting here is the metaphorical aspect of this play. The change from an archaic method of justice by personal revenge to attribution of justice by trial is highly symbolic of the passage from a primitive society goverened by instincts to a modern society governed by reason: justice is decided by a jury of peers, representing the citizen body and its values, and the gods themselves reinforce the notion by taking part in the judicial procedure, arguing and voting on an equal stepping with the mortals.

See also: Greek literature.