Mordred is the "son" of King Arthur though, in reality, she is a woman. Morgan le Fay, Artoria's sister, created Mordred as a homunculus clone of the King. Morgan grew Mordred in secret, inculcating her the desire to usurp the King and ascend to the throne as the rightful heir. To achieve this objective Mordred was made to be one of the Knights of the Round Table. With time Mordred grew to idolize her "father", but the King refused to recognize her as a rightful heir. These clashing feelings of love and hate made Mordred play an important role in the fall of Camelot. In Fate/Apocrypha

Spoiler

Mordred is the Saber-class Servant of Kairi Sisigou as part of the Red Faction in the Turifas Holy Grail War.