The Hirudin was a composite lifeform of many leech like organisms. It was in command of the Red Caps and gave orders to Captain Heyward, the leader of the red coats who were attacking Black Donald.
When Heyward failed it, the Hirudin creatures attacked him, draining his blood and infesting his body to use as a disguise. (AUDIO: City of Spires)
The Sixth Doctor later discovered that he was a fictional character who only existed in the Land of Fiction. (AUDIO: Legend of the Cybermen)