Mordred is King Arthur's son from a relationship with Margawse. He's not born from Arthur's marriage, so he's seen as illegitimate. Mordred's fate is to eventually kill his father, Arthur. When Arthur learns of this destiny, he tries to have Mordred killed as a child by sending all babies born in his birth month on a ship to be sunk, but Mordred survives.
As Mordred grows up, he takes part in killing Lamorak, a noble knight. Eventually, he turns against Arthur while Arthur is away fighting Launcelot. Mordred's existence shows how the book is fascinated with royal bloodlines and fate, especially when mixed with competition and jealousy. It seems inevitable that a son will rebel against his father, according to the story.
Mordred's story adds drama and tension to the legend of King Arthur. His conflicted relationship with his father reflects the complexities of power, family, and destiny in the Arthurian tale.