Medusa: A Tiger by the Tail - The Four Lords of the Diamond, #4

Aliens are spying on Earth from one of the four worlds that make up the Warden Diamond. But it is impossible to send agents to any of the four worlds because of unique microscopic symbionts that make it impossible for visitors to leave the Diamond. Seeking a unique solution, each of the four worlds making up the Diamond is sent a person whose mind has been stripped of everything and who is now controlled by an agent of the Confederacy.
The frozen world of Medusa is the last of the four worlds to be explored. But are Tarin Bul and his controller up to the job? The secret powers of the hated dictator, Lord Ypsir, are far more extensive than had been anticipated and may prove too much for them. Can the Agent With No Name save the mission?This is the fourth and final book in the Four Lords of the Diamond series.