Atreus promised Artemis that he would sacrifice his best lamb to her. Upon finding a golden lamb, however, he gave it to Aerope to hide. She, in turn, gave the lamb to Thyestes, who asked Atreus if the person with the golden lamb should be king. Atreus agreed and Thyestes seized the throne.
Atreus eventually retook the throne through similar treachery (see Atreus) and the brothers' rivalry, part of the curse of Tantalus, continued for two more generations (see Agamemnon and Orestes).