Clytaemnestra in Greek mythology was daughter of Leda and Tyndareus, king of Sparta. She was unhappily married to the king of Mycenae, Agamemnon, and eventually murdered him with the assistance of Aegisthus, his cousin. She was the mother of Iphigenia, Electra, and Orestes (the former being sacrificed by her father to Artemis and the latter two conspiring to kill their mother). She was later killed by her son and her husband's nephew (Pylades), aided by Electra.
Warning in The Odyssey
The dead Agamemnon as a shade in the Land of the Dead tells Odysseus of that story of his death, and makes an analogy of Clytaemnestra, telling Odysseus not to make a big deal of his return to Ithaca, in case Penelope his wife would also be unfaithful, having not seen Odysseus for almost twenty years.