The Duel between Arthur and Elyanoccurred when Elyan was possessed by the spirit of the shrine boy who wanted him to avenge his death. When the spirit told Elyan that he wanted peace, but in order for him to get that the King (Arthur) needed to be killed. Elyan, who is now under it's power, goes to Arthur's chambers at night and attacks him. But although he catches him off guard, the King saves himself with a chair. The guards come in, which forces Elyan to flee.
Later Elyan makes a second attempt to end Arthur's life. Armed with a crossbow, he kills three knights. He fires at Arthur, but he dodges the shot. Elyan is now completely under the control of the spirit, speaking with it's voice rather than his own. He engages in a sword fight with Arthur, who once again defeats him. Yet now that he has heard the shrine boy's voice instead of Elyan's, he starts to realise that he is possessed like Merlin said, and allows him to flee (A Herald of the New Age).