Nessus and Deianeira. To find out more about the centaur Nessus and Deianira, the wife of Heracles, see this Wikipedia article: link; for information about the image: image source.
This painting by Arnold Böcklin shows Deianeira, the wife of Heracles, putting up a serious fight against the centaur Nessus. As you can see, Nessus is trying to kidnap Deianeira, but Heracles arrives on the scene and shoots the centaur with an arrow dipped in the poisonous blood of the hydra. Before he died, though, Nessus conceived a way to get his revenge: he told Deianeira that if she took his blood and semen, she could later use it as a love potion if Heracles ever strayed. Later on, when Heracles did fall in love with someone else, Deianeira got out the vial in which she kept the blood of Nessus and smeared it on Heracles's' tunic, thinking it would make him fall in love with her again. Instead, it brought about his death. In despair, Deianeira killed herself.
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