Thyestes seduced the wife of Atreus, Aerope. Atreus pretended to forgive his brother and invited him to a feast at which he served Thyestes a slightly different (but more grimly appropriate) feast from the one his grandfather had served the gods. Instead of serving his children, Atreus served his brother, Thyestes' own, cooked children. This time there were no gods to reverse the outrage and bring the innocent children back to life.
In revenge, Thyestes produced a son, Aegisthus, who murdered Atreus.