The second wife of Heracles, in Greek legend, and the one who unwittingly brought about his death. She was the daughter of Oeneus, and was wooed by the river god Achelous, who horrified her. Much to her relief Heracles also came along and contested with Achelous.
On their way home as newlyweds, they were crossing the river Evenus. Crossings were handled by Nessus, a randy old centaur. When he was half way across with her he offered violence. Heracles, seeing this, slew Nessus with his arrows dipped in the blood of the Hydra, which he had slain as part of his labours. This made the poison invincible.
Nessus, while dying, told Deianira that his blood would make a very effective love potion if she was ever worried that Heracles was getting a bit on the side. She was to keep it out of the way of sunlight and sacred fire, and anoint a robe when she needed it. She believed him (hey, she was just a kid), and somehow contrived to take his blood and keep it, though whether they were still in midstream now or had crossed to the other side, I cannot tell you.
This story is told by Apollodorus, and also by Sophocles in his play Trachiniae (The Women of Trachis).
Heracles went on many adventures after that, including being captured by Queen Omphale in Lydia and being forced to wear women's costume and do women's work for a year; then he went on a war against King Eurytus of the city of Oechalia. Here he met and loved Iole, the daughter of Eurytus. Sophocles has a messenger explain that this is why he sacked the city; in fact the dispute between the messenger and the herald over the truth of Heracles' motives plays an important role in the drama.
Meanwhile Deianira languished at home, waiting for him. They had had many sons, the eldest of whom was Hyllus. He is apparently not yet of age. She and Heracles both knew of a prophecy that after fifteen months of absence he would return from his travails and finally find peace. The fifteen months have just expired at the opening of Trachiniae, of which she is the protagonist. The chorus are her friends the women of Trachis.
In Trachis, where they live, Heracles is preceded by captives from Oechalia, including Iole, whom Deianira recognizes as one of high stature. She learns from the messenger who she is, and is saddened, but not surprised, for this is by no means the first dalliance her husband had had. But this time the fifteen months' absence must have made a difference, for she decides to try the love ointment that Nessus had given her. She anoints a robe and sends it to him to wear while he is sacrificing to celebrate victory.
Hyllus returns, having seen his father's dying agony with his own eyes, and curses his mother. She has already seen what would happen: the piece of wool she had used to anoint the robe had disintegrated into ashes when she had exposed it to the sun. She has already realized how Nessus had deceived her. With Hyllus' news, she goes indoors, and kills herself. Sophocles has her do it with a sword, though Apollodorus makes it the more usual hanging.
In all this she shows no bitterness to Iole, virtually no jealousy. She knows that she is older, and Iole is young, and Heracles is lusty, and that is how things are.
There is a Roman version of the play by Seneca, Hercules Oetaeus, in which she is made out as a virago.