Mordred is the greatest villain of Arthurian legend, almost a predestined villain, one who is tied up with a fate King Arthur cannot escape. In the earliest versions of the legend, Mordred is usually Arthur's nephew, son of Arthur's half-sister Morgause, but over time, he became Arthur's son, born of incest by either Morgause or Morgan le Fay. His traditional birthdate is May 1st. When Arthur learned the woman who had seduced him was his sister and she had born his child, he ordered all baby boys in the kingdom to be slaughtered, but Mordred managed to survive.
Eventually, Mordred reached manhood, and because Arthur and Guinevere could not have children, Arthur allowed him to come to Camelot. When Arthur goes campaigning against Rome, Mordred seizes the kingdom, eventually resulting in the Battle of Camlann.
Like Morgan le Fay, Mordred has been made more sympathetic by later writers, which may even be in keeping with his true nature. As explored in my book King Arthur's Children, several Welsh and Scottish traditions speak well of him and even suggest that Arthur was the true villain.