Clytemnestra
Clytemnestra was the daughter of Tyndareus and Leda. Her half-brother Pollux and half-sister Helen (later known as Helen of Troy) were the offspring of Leda and the Swan (Zeus), so they were immortal while she and her brother Castor were not. One wonders what dinnertime conversations were like when those four were growing up.
Clytemnestra and Helen married the brothers Agamemnon and Menelaus, respectively, and Clytemnestra became Queen of Mycenae.Clytemnestra and Agamemnon had one son, Orestes, and three daughters, Iphigenia, Electra, and Chrysothemis. Chrysothemis seems to have been pretty much of a non-entity and does not feature much in the myths. Iphigenia was lured to Aulis by her father under pretext of marriage to Achilles, and there offered for sacrifice. Although Iphigenia was saved at the last instant by Artemis, Clytemnestra never forgave Agamemnon for the attempt.
While Agamemnon was off for ten years at Troy, Clytemnestra allowed his cousin, Aegisthus, to move in with her. The two plotted against Agamemnon, Aegisthus as part of the continuing hostility between the family of Thyestes and the family of Atreus, and Clytemnestra as revenge for the attempted sacrifice of Iphigenia. It appears that Electra thought their plots against Agamemnon might include the murder of Orestes (child murder being a tradition in the House of Atreus) and she had him smuggled away to safety during this time.
On Agamemnon's return, Clytemnestra and Aegisthus murdered him in the bath, with Clytemnestra herself striking the fatal blow. The two then married and Aegisthus became king of Mycenae, as his father had briefly been king before him. They attempted to foreclose revenge by Electra or her children by marrying her off to a peasant, but this was unsuccessful as when Orestes turned twenty, he was ordered by Apollo to avenge his father's death by executing his murderers. Reluctantly, but with the aid of Electra, he did so, killing his own mother and thus bringing the wrath of the Furies on him.