Margwyn was Queen Mab's most trusted advisor in Avalon. He was manipulated by Cavis and Gandar to help start a war as a distraction so they could reach the sleeping Constantine, who maintained the reality of Avalon, in order to get him to attempt to kill Compassion.
He saved the Brigadier from being killed by Gandar, but was shot and died from his injuries. The amulet he wore, which had been passed down through his family for centuries, was a failsafe to return Constantine to sleep if he was ever awoken. Mab was able to use it to do so, restoring Avalon. (PROSE: The Shadows of Avalon)