The Eumenides (Εὐμενίδες, Eumenides; also known as The Kindly Ones) is the final play of the Oresteia, in which Orestes, Apollo, and the Erinyes go before Athena and eleven other judges chosen by her from the Athenian citizenry at the Areopagus (Rock of Ares, a flat rocky hill by the Athenian agora where the homicide court of Athens later held its sessions), to decide whether Orestes's killing of his mother, Clytemnestra, makes him guilty of the crime of murder.
Orestes is tormented by the Erinyes, or Furies, who are chthonic deities that avenge injustice, matricide in Orestes' case. He, at the instigation of his sister Electra and the god Apollo, has killed their mother Clytemnestra, who had killed their father, King Agamemnon, who had sacrificed his daughter and Orestes's sister, Iphigenia so that his fleet could set sail for Troy. Orestes finds a refuge and a solace at the new temple of Apollo in Delphi, and the god, unable to deliver him from the Erinyes' unappeasable wrath, sends him along to Athens under the protection of Hermes, while he casts a drowsy spell upon the pursuing Erinyes in order to delay them.
Clytemnestra's ghost appears "exactly how or from where is uncertain ... noteworthy is the poet's bold inventiveness in presenting her as a dream to a collection rather than to a single individual", to the sleeping Erinyes, urging them to continue hunting Orestes. "As the first of them begins to awake the ghost departs". The Erinyes' first appearance on stage is haunting: they hum in unison as they slowly wake up, and seek to find the scent of blood that will lead them to Orestes' tracks. An ancient legend says that on the play's premiere this struck so much fear and anguish in the audience, that a pregnant woman named Neaira suffered a miscarriage and died on the spot.
The Erinyes' tracking down of Orestes in Athens is equally haunting: Orestes has clasped Athena's small statue in supplication, and the Erinyes 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 twelve Athenians to join her in forming a jury to judge her supplicant. Apollo acts as counsel for Orestes, while the Erinyes act as advocates for the dead Clytemnestra. During the trial, Apollo convinces Athena that, in a marriage, the man is more important than the woman, by pointing out that Athena was born only of Zeus and without a mother. Athena votes last and casts her vote for acquittal; she does so before the votes are counted. After being counted, the votes on each side are equal, thus acquitting Orestes as Athena had earlier announced that this would be the result of a tie. She then persuades the Erinyes to accept the verdict, and they eventually submit. Athena then leads a procession accompanying them to their new abode and the escort now addresses them as "Semnai" (Venerable Ones), as they will now be honored by the citizens of Athens and ensure the city's prosperity. Athena also declares that henceforth tied juries will result in the defendant being acquitted, as mercy should always take precedence over harshness.