One of Arthur’s knights in Renaut de Bâgé’s Le Bel Inconnu who fights in the Castle of Maiden’s tournament and is defeated by Guinglain, the son of Gawain.
This does not appear to be the same character as Arthur’s nephew, because he is named as a king, because he is given a brother named Segures, and because no mention is made of the relationship between Mordred and Guinglain. On the other hand, Segures is similar to Seguarades, who is Mordred’s foster-brother in the Post Vulgate Merlin continuation.
See also
Mordred the Traitor | The Legend of King Arthur
Source
Le Bel Inconnu | Renaut de Bâgé, 1185–1190