Nessus was a centaur in Greek mythology, who was killed by Heracles and this eventually led to the hero's demise. He was the son of Centaurus, who was the father of all creatures known as the centaurs.
Nessus is well known for his part in the story of the Shirt of Nessus. He was a ferryman, and one day, he had to carry Deianeira, wife of Heracles, across the river. After they crossed the river, Nessus tried to have sex with her, but Heracles watching from the other riverbank, shot an arrow straight into Nessus' chest. Before he drew his final breath, Nessus told Deianeira that his blood would ensure that her husband would be faithful to her in eternity. Deianeira believed him and collected some of the centaur's blood.
Years later, Heracles fell in love with Iole, and Deianeira, afraid that he might abandon her, dipped a shirt or tunic into the centaur's blood and gave it to him as he was leaving for a gathering. After he left, she accidentally spilled some of the blood on the floor, where it started fuming. She then realised that it was in fact poison and tried to stop Heracles from wearing the shirt. However, the hero had already worn it and his flesh had started burning; in deep pain, he threw himself in a funeral pyre and died.