The Zirconian Emperor was the leader of the Zircon.
After exhausting a planet that the Emperor had grown tired of, he gave the order to destroy a dying planet in an attempt to use a galactic cyclone to wipe out the people of Earth to provide a new home. He met with his commanders and, near the Moon, he sent attack ships to be ready to kill any survivors.
The Emperor was presumably killed when the Zirconian fleet was destroyed by the Third Doctor and Sarah Jane Smith. According to another account, it was by the Fourth Doctor and Joan Brown. (COMIC: Doomcloud)