The duel between Arthur and Lancelot's Shade occurred after Arthur saw Lancelot kissing Guinevere, who Morgana had secretly enchanted with a love spell.
Events
In order to stop Gwen from becoming Queen, Morgana raised Lancelot from the dead as a Shade and used a love spell to rekindle Gwen's love for him. The night before she and Arthur were to be married, the Shade Lancelot arranged for Gwen to meet him in the council chambers and they shared a kiss. Unfortunately, they were seen by Arthur, who had been directed there by Agravaine.
Enraged, Arthur attacked Lancelot and they began to duel. Arthur elbowed Lancelot in the face and Lancelot responded by shoving Arthur into a column, causing both to lose their swords. Arthur shoved Lancelot and he slid close enough to his sword to grab it. Lancelot tried to attack Arthur, but Merlin intervened with his magic and disarmed him. Arthur retrieved his own sword and charged Lancelot, but Gwen placed herself between them, forcing them to stop the duel.
Arthur later had Gwen and Lancelot arrested and imprisoned in the dungeons. Gwen was exiled from Camelot for her betrayal and Lancelot, acting under Morgana's orders, committed suicide (Lancelot du Lac).