The sorceress, Nimueh, attempted to distabilise attempts at peace between Mercia and Camelot by framing Bayard, king of Mercia, for the attempted assassination of Arthur Pendragon, prince of Camelot, with a poisoned goblet. Bayard's forces were poised to attack Camelot after he was imprisoned by King Uther, but the truth was discovered by Gaius in time to prevent war and Bayard was released (The Poisoned Chalice).
After King Odin's son was killed by Arthur in a duel, the king became determined to get revenge. He conspired to have Arthur Pendragon killed by the notorious assassin Myror. Myror always got his prey and was well known for it. He first tried to acquire Arthur in the woods near Camelot because he had heard that the prince would be out hunting. When the assassin realised that Arthur was nowhere in sight he hastily returned to Camelot to investigate the matter. He saw the young Pendragon in Gwen's home and tried to kill him there, but he was seen by the royal guard and thus forced to flee. He finally discovered that Arthur was partaking in the joust, albeit incognito. Myror murdered the last knight Arthur was supposed to fight and took his place. He planned to kill his prey by using a pike hidden in his lance. Though he managed to wound Arthur, Merlin intervened by breaking a strap on Myror's saddle. This distraction gave Prince Arthur the chance to slay his would-be assassin (The Once and Future Queen).
Odin later sent a second assassin to kill Arthur, the Gleeman, under the guise of a circus master who arrived in Camelot to provide entertainment at Arthur's birthday feast. Ultimately, he failed to kill Arthur but was responsible for the death of Uther Pendragon, whom he mortally wounded when trying to kill Arthur. However, the attempt cost him his own life as well (The Wicked Day).
In her plan to kill Arthur, Morgana summoned a multi-headed Fomorroh to enslave Merlin's mind, driving him to kill Arthur. After several of his attempts on Arthur's life failed, Gwen and Gaius managed to catch Merlin. Gaius removed the Fomorroh head and killed it, but asecond one grew in its place. After stopping yet another attempt to kill Arthur, Gaius paralysed the snake's head, which temporarily gave Merlin back his free will. Under the guise of Dragoon the Great, he traveled to Morgana's hut, found the mother beast, and killed it after prevailing in a close battle with Morgana. Back in Camelot, Gaius removed the head still left in his neck (A Servant of Two Masters).