Thyestes is a son of Pelops and Hippodameia. He and his brother Atreus killed Pelops' bastard son, Chrysippus, and they were exiled. Invited by the king Sthenelus, the two brothers fled to Mycenae.
In Mycenae, Atreus purchased a wife for himself, named Aerope. She bore him two sons, Agamemnon and Menelaus. She did not love Atreus, however, and was having a secret affair with Thyestes. While together, she told Thyestes of Atreus' secret treasure: a golden fleece from a sheep he was supposed to have sacrificed to Athena.
When Sthenelus' son Eurystheus died, Atreus and Thyestes were the two principal contenders for the throne. Thyestes proposed that the throne should be given to the man, who could produce a golden fleece. While trying to conceal his delight over such incredible luck, Atreus agreed. What he did not know, however, was that his wife Aerope had already stolen the fleece and given it to Thyestes. When Thyestes produced the fleece, he was given the kingdom.
Atreus knew he had been tricked, but he had no proof. On the advice of Hermes, Atreus proposed that he should be given the kingdom, if he could make the sun reverse its course. Thinking his brother mad, Thyestes agreed to this impossible test. Zeus, however, favored Atreus, and caused the sun to in fact reverse its course. Atreus was given the throne, and he then banished his brother Thyestes.
Later, when Atreus learned exactly how Thyestes had obtained the fleece, felt that banishment was far too lenient a punishment. On the pretence of forgiveness, Atreus invited Thyestes back. When he returned, Atreus secretly killed Thyestes' children. At a banquet that night, Atreus served them to Thyestes during the course of the meal. At the end of the meal, Atreus showed Thyestes the heads of his children, and let him know the truth about his meal. Then Atreus once again banished Thyestes.
Thyestes, seeking vengeance, consulted an oracle. The oracle said that Thyestes should have a child with his own daughter, Pelopia, who lived in Threspotia. Thyestes did as he was told. Pelopia, who was now pregnant, stayed in Threspotia, and Thyestes went to Lydia.
In Mycenae, a terrible famine was raging as a result of Atreus' crime. He consulted an oracle, which told him to bring Thyestes back to Mycenae. While searching for him, Atreus came to Threspotia. There, he saw the beautiful Pelopia, and asked for her to be his wife. The king agreed, and Pelopia returned to Mycenae with Atreus. When her child was born, she had him exposed, but he was rescued by some shepherds. When Atreus found out about the boy, named Aegisthus, he had him brought to the palace and raised as one of his sons.
Eventually, Thyestes was captured and brought to Mycenae. Atreus had him placed in a cell, and ordered Aegisthus to kill him. When the boy approached, Thyestes recognized his sword, which he had given to Pelopia years before. Aegisthus consulted his mother Pelopia, to see if this was true. When Pelopia realized that she had slept with her own father, she killed herself. Aegisthus then freed his father Thyestes and killed his step-father Atreus.
Thyestes was now the king of Mycenae, but not for long. The sons of Atreus, Agamemnon and Menelaus, returned and reclaimed the throne. Thyestes and Aegisthus were forced to flee. Thyestes would never again return to Mycenae. But his son Aegisthus would eventually have his revenge, by murdering Agmemnon.