Magnus III was the king of Elbyon in 2991, and husband to Queen Leonora. He joined forces with the First Doctor to fight against the sorcerer Marton Dhal, and the Earth Empire, who both wanted to take control of the planet.
King Magnus, Marton Dhal, and the Earth Empire all vied for the master control to the planet's reality, which had become known as Merlin's Helm. A war ensued, which culminated in the defeat of the Empire and Dhal after the Cephlie volunteeringly took control of the Helm and wiped themselves out.
Afterwards, Admiral Nyborg and Magnus set to peace talks, with suggestions that the struggling Earth Empire might seek to house some of its citizens on the sparsely populated planet. (PROSE: The Sorcerer's Apprentice)