Merdeen was the leader of the guard in an underground kingdom, the UK Habitat. He was responsible for contacting Drathro via video screens. To show his commanding status his uniform and helmet were black.
Secretly he was helping people escape from the underground kingdom to the Tribe of the Free. Grell discovered this but was killed by Merdeen before he could take action. He helped the Sixth Doctor overthrow Drathro and then left the kingdom and entered the outside world. (TV: The Mysterious Planet)