Clytemnestra was the daughter of Leda—yes, the same Leda that was raped by Zeus in the form of a swan—and Tyndareus, a Spartan king. She was the wife of Agamemnon, a hero of the Trojan war. According to Euripedes, he was her second husband—and had murdered her first, forcing her to marry him. Whilst trying to return from the Trojan war, the gods turned the winds against Agamemnon, demanding that he sacrifice his daughter—Clytemnestra’s daughter—Iphigenia, to turn the winds back in his favor. Agamemnon convinced Clytemnestra to send Iphigenia to him, under the lie that she would be married to Achilles, then did as the gods demanded.
When Clytemnestra learned that her husband had sacrificed Iphigenia to the gods, she was wild with grief. She was also beyond furious. She had taken Agamemnon’s cousin Aegisthus as her lover, and when Agamemnon returned home—with his concubine, the doomed prophetess Cassandra in tow—Clytemnestra and Aegisthus plot their deaths. She waits until Agamemnon is at his most vulnerable—taking a bath—then ensnares him in a net and stabs him repeatedly until he dies. (At least, according to Aeschylus’s Oresteia. Euripedes mentions others doing her bidding.)
Cassandra, meanwhile, is sitting in the chariot doing her best to get someone, anyone, to go in and stop Agamemnon from being murdered, which she’s seen in a vision, but because of her curse from Apollo (who she’d spurned) no one listens to her. Knowing she’s fated to die, she walks into the palace and allows herself to be slain. At least, again, in the Oresteia. In some versions, she escapes.
Eventually Clytemnestra has to pay for her sins—at the hand of her own son, Orestes.
I don’t know why I’ve always had such a fascination with Clytemnestra. Maybe because I feel so bad for her… her story is truly tragic. Not that she handles it in the best of ways, but… you can’t help but feel a little sorry for her, despite everything she does.