Eteocles was born in the palace of Thebes into a house already drowning in doom. His grandfather Laius had been told by the oracle of Delphi, the sacred shrine of Apollo, the god of prophecy, that his own son would kill him, and everything done to prevent that prophecy had only carried it out: the infant Oedipus, pierced through the ankles and left to die on Mount Cithaeron, had survived, had grown up in Corinth, had killed Laius in a roadside quarrel without knowing him, and had come to Thebes, saved the city from the Sphinx, and married the widowed queen Jocasta, his own mother. When the truth came out, Jocasta hanged herself and Oedipus blinded himself with the golden pins of her robe. Into that ruin Eteocles was born, one of four children, with his brother Polynices and his sisters Antigone and Ismene. The Greeks told two things about the brothers' youth: that they grew up in a palace where their father was a living curse, and that as young men they mistreated him shamefully, shutting the blind old man away in the inner rooms, serving him from vessels of cheap metal, and laying hands on his golden staff, the one token of kingship the broken man had left. From that mistreatment grew the curse that would kill them both.

One influential mythographic tradition says Eteocles and Polynices agreed to rule Thebes in alternate years. The sources do not preserve one fixed sequence. Pseudo-Apollodorus explicitly records two versions: some said Polynices ruled first and handed the kingdom over, while others said Eteocles ruled first and then refused to surrender it. Diodorus gives Eteocles the first year and makes his refusal the immediate cause of Polynices' exile. The alternating-year compact should therefore be presented as a major tradition, not as the only ancient account.

The curse of Oedipus is also transmitted in more than one form. Sophocles' Oedipus at Colonus makes the exiled father curse his sons and foresee their mutual destruction, while other traditions place the family conflict differently. The curse became one of the strongest explanations for why a political dispute had to end in fratricide, but the surviving Theban cycle is a collection of variant poetic traditions rather than one continuous canonical biography.

Polynices carried his wrong to Argos, the city of the Danaans across the gulf, and there he found his war. The king of Argos, Adrastus, had received an oracle that he would marry his daughters to a lion and a boar, and when Polynices and the exiled prince Tydeus arrived at his door on the same night and fought each other like beasts over the shelter of the porch, Adrastus saw the oracle fulfilled and gave them his two daughters: Argia to Polynices, Deipyle to Tydeus. Around these marriages the expedition called the Seven Against Thebes assembled: Adrastus himself; Tydeus, the fierce son of Oeneus; Capaneus, a giant who mocked the gods; Hippomedon, hard as his name; Parthenopaeus, the young son of the huntress Atalanta; the seer Amphiaraus, who could see that not one of the Seven would return and tried to refuse the war; and Polynices, whose claim was the cause of all. Amphiaraus hid himself away to escape his fate, and his wife Eriphyle was bribed with the necklace of Harmonia, a jewel the gods had cursed at its making, to send her husband out to die. The oldest poem of Greece already knows the shape of this gathering: in the Iliad, King Agamemnon reminds the young hero Diomedes that Diomedes' father Tydeus once travelled in Polynices' company through the cities, mustering allies for the war on Thebes.

They came against Thebes with seven champions, and the city had seven gates, and the poet Aeschylus built his whole play on that arithmetic. The drama is almost bare: Eteocles alone stands against the panic of the citizens, walking the walls and posting a defender at each gate as the enemy draws near with their terrible shields, each blazoned with a boast. To the gate where Tydeus raves, with the full moon and a sky of silver stars on his shield, he answers a seasoned lord; to the gate where Capaneus swears he will sack the city even if Zeus hurls fire to stop him, he answers a man of weight; to the gates of Hippomedon and Parthenopaeus and the rest, he answers in turn, one by one, the king disposing his men like a player laying out the last pieces of a losing game. When the messenger at last names the seventh champion, Polynices, standing at the seventh gate with justice, he says, on his side, Eteocles falls silent, and in that silence the audience hears the curse close like a door. A chorus of Theban women begs him not to go, not to be his brother's executioner and his own. Eteocles answers that a man cannot flee what is fated, that a king who abandoned his post before the spear would be no king at all, and that his father's curse is already waiting at the gate beside his brother. He takes up his spear and goes, and the play ends with the messenger's report: at the seventh gate the two sons of Oedipus drove their spears through each other at the same moment and divided their inheritance with iron, exactly as the curse had said, and the city of Thebes survived them.

The brothers' mutual death ends the immediate attack on Thebes. Later works develop the aftermath differently. Sophocles' Antigone famously makes Creon honour Eteocles as defender of the city while forbidding burial for Polynices, and Antigone's defiance creates the next tragedy. Other sources preserve different political arrangements after the war, including the succession of Eteocles' son Laodamas.

Ancient writers repeatedly returned to the brothers because neither claim is simple. Eteocles can be the ruler who breaks an agreement and also the king who defends Thebes from an invading army. Polynices can possess a claim to rule and still bring foreign forces against his own city. Aeschylus, Sophocles, Euripides and later epic writers use different versions of the family story, so modern readers should resist flattening the cycle into a single moral verdict.