Canon Background
https://awoiaf.westeros.org/index.php/Mandon_Moore

In-Campaign
He was a combatant in the sunset melee at the tournament celebrating the royal marriage of Robert Baratheon and Cersei Lannister. He was the last man standing thus was declared winner of the contest.

At the Spring Tourney of 288 AC Ser Mandon accompanied Lord Arryn home to the Vale. He served as the ‘Champion of the Sun’; his reputation appeared to deter many would be challengers. That was true until Ser Darron’s turn. He chose to challenge Kings Guard and his boldness paid off. Ser Darron managed to defeat Mandon, making many a man who had bet on him as a long shot a small fortune. Darron forewent the customary ransom and instead asked Mandon’s forgiveness for a jape he had made at Mandon’s mother’s expense… and also for a tour of the White Tower.