Master of Karn

The Master of Karn was the surviving member of his species. He didn't believe that the Doctor didn't want to use the crystals, but just give them to the Time Lords. (AUDIO: Seven Keys to Doomsday)

Behind the scenes
According to Simon Jones, who played this part in the original stageplay, the script at one point described the Master of Karn as "twelve feet of cobwebby malevolence". (BFX: The Haunting of Malkin Place)