The duel between Arthur and Elyan occurred after the spirit of a Druid boy possessed Elyan to avenge his death.
Events[]
Elyan made two attempts to kill Arthur after he became possessed. On the first attempt, he ambushed Arthur in his chambers during the night, but Arthur was able to hold off his attacks with a chair until the guards arrived and forced Elyan to flee.
On the second attempt, Elyan attacked Arthur in the council chambers with a crossbow. Arthur dodged the shot and took a sword from the one of the guards Elyan had killed, after which Elyan abandoned the crossbow and picked up his own sword and the two began to duel.
Though Elyan's skills were somewhat hindered by his possession, Arthur's reluctance to kill Elyan allowed him to hold his own. After an intense fight, Merlin blasted the chamber doors open with his magic and the explosion distracted Elyan long enough for Arthur to disarm him by throwing him to the floor. Arthur then held Elyan at swordpoint, but ultimately chose to let him escape because he now believed Merlin's theory that he was being possessed (A Herald of the New Age).