From The Art and Popular Culture Encyclopedia
Nessus is known for his role in the story of the Tunic of Nessus. After carrying Deianeira, the wife of Heracles, across the river, he attempted to rape her. Heracles saw this from across a river and shot a Hydra-poisoned arrow into Nessus's breast. As a final act of malice, Nessus told Deianira, as he lay dying, that his blood would ensure that Heracles would be true to her forever.
Deianira foolishly believed him. Later, when her trust began to wane, she spread the centaur's blood on a shirt and gave it to her husband. Heracles went to a gathering of heroes, where his passion got the better of him. Meanwhile, Deianeira accidentally spilled a portion of the centaur's blood onto the floor. To her horror, it began to fume by the light of the rising sun.
She instantly recognized it as poison and sent her messenger to warn Heracles but it was too late. Heracles lay dying slowly and painfully as the shirt burned his skin—either in actual flames or by the heat of poison. He died a noble death on a funeral pyre of oak branches, and was taken to heaven by Hera and welcomed amongst the gods for his heroic exploits.
A similar theme appears in certain versions of the story of Medea.