The King Vampire was significantly more powerful than the other vampires. As with all vampires, his heart needed to be destroyed completely for him to die.
The King needed blood as his primary food source, which he drank by sending swarms of bats to devour it from sacrifices prepared by his servants, as well as directly. His servants claimed that he fed on the soul as well as the flesh. (TV: State of Decay)
The Yssgaroth waged a great war against the Time Lords, who defeated them all with Bowships created on the orders of Rassilon. Only the swarm-leader survived. (PROSE: Interference) He travelled into E-Space, and summoned the human spacecraft the Hydrax there to use its crew as servants.
The ship landed on a planet where it became castle-like in appearance over time. A community grew around it. The King called three humans from the Hydrax telepathically, and made them into Vampires, to serve him and rule over generations of the other humans. Peasants were selected to be fed to the King Vampire, including Karl, son of the head villager Ivo. The fuel tanks of the Hydrax were filled with blood, which was fed to the King through tubing. He lay dormant under the ship for a thousand years, growing in power.
When the Time of Arising came, the King Vampire rose from the ground. The Fourth Doctor launched a scout ship from the Hydrax. The steel hull pierced the King's heart as it descended. (TV: State of Decay)