Once upon a time, the crimes of King Atreus of Mycenae, had caused the fields to become barren. To remove the barrenness, an Oracle was consulted, who advised to recall Thyestes, the banished brother of Atreus.
Atreus travelled in search of Thyestes and arrived at the court of King Thesprotus, where for the first time, he saw his niece Polypia, the daughter of Thyestes. Thinking Polypia to be the daughter of Thesprotus, Atreus asked for her hand and Thesprotus agreed. At that time, Polypia had Aegisthus in her womb, conceived from her being raped by an unknown person. Not knowing that Polypia was pregnant, Atreus brought her to Mycenae, where she gave birth to Aegisthus. Atreus raised Aegisthus as his own son.
Years passed and Atreus, unable to find Thyestes, sent his two sons Agamemnon and Menelaus, also known as the Atrides, to seek guidance from the Oracle at Delphi. By chance, at Delphi, the Atrides met Thyestes and they brought him to Mycenae. Atreus threw Thyestes in a dungeon. Since, Atreus had recalled Thyestes back, he thought the oracle was fulfilled. Not needing Thyestes anymore, Atreus told Aegisthus, whom he considered as his third son, to slay Thyestes.
Aegisthus approached Thyestes in the prison and drew his sword. Seeing the sword, Thyestes recognized it as his own sword. He asked Aegisthus about the sword and Aegisthus replied that it was given by his mother. Thyestes requested to call Pelopia, who came and said that the sword belonged to the person, who had raped her. Thyestes told them that it was his sword. The aghast Pelopia, realizing that her own father, was the father of her own son, snatched the sword, thrust it deep in her breast and died. Aegisthus took the bloody sword to Atreus, and showed it as proof of killing Thyestes. Later, when Atreus was at a sacrifice, Aegisthus killed Atreus. Thyestes became the King of Mycenae and the Atrides were exiled.
Excerpt from the book “Once Upon A Time-II: 150 Greek Mythology Stories” by Rajen Jani