Aegisthus

In Homer he is the cousin who took Agamemnon’s wife and his throne while the king was at Troy, then murdered him at the homecoming feast. He ruled Mycenae for seven years before Orestes returned and killed him. [SPOILER: His death at Orestes’s hand is the parable Athena and Nestor and Agamemnon’s own ghost all hold up for Telemachus, the model for what a son does when his father is gone and the wrong men sit at his table.] The retelling keeps the parable but does not name the man. He becomes “her secret lover” in Agamemnon’s account. The reasoning is the same as with Clytemnestra: keep the focus on the shape of the betrayal, not on building out a parallel cast. The Orestes parallel still works without naming the killer.