Mordred makes his only appearance in the Shining Knight series in Adventure 153 (June 1950), and conveniently happens to be in the right place when Bellerophon (from Greek mythology) winds up transported to Camelot by fumes from a magic cave.
That sounds dumb, I know, but the story is better than most of the Shining Knight’s tales, and is by Frazetta to boot.
Mordred hates Sir Justin, of course. No need to give any reason behind that, Mordred is already the villain of Arthurian legend. He lies to Bellerophon and convinces him to battle the Shining Knight.
As both combatants have flying horses it makes for quite the battle. In fact, the two are so evenly matched that neither actually triumphs. Merlin figures out when and where Bellerophon is from and sends him back to his own time.
Mordred sulks, but is not punished in any way.