Captain Shannon (The Sorcerer's Apprentice) was an Earth Empire secret agent. He went down to Avalon to investigate the planet with Vincent Monadno, Ivanov and Jen Komati.
In fact, the Earth Empire had discovered that a nearby nebula was caused by a weapon from the planet having fired on and destroyed a star centuries ago. They sent Shannon to retrieve that weapon. He was with the expedition that found Merlin's Helm, the control device for the planet's nanotechnology. He was not able to get it from the Avalonians at that time.
Later, when the First Doctor, Anni Glassfeather and Kildenny Odoyle were magically fighting Gramling to prevent his use of Merlin's Helm, Jen Komati grabbed it and threw it to Ian Chesterton. Shannon shot her for not giving it to him. He was then killed by Sir Bron. (PROSE: The Sorcerer's Apprentice)