The Fisher King was an alien warlord who conquered Tivoli and ruled it for ten years. However, the planet was eventually liberated by the Arcateenians. He apparently died and his body was transported to Earth in 1980 by the Tivolian funeral director Albar Prentis. However, he was still alive and planned for his people to recover him by turning people into ghosts transmitting coordinates to Earth. He first shot and killed Prentis, who would become a ghost in 2119. His next victim was Alice O'Donnell, after he found her hiding in an abandoned building. He was tricked by the Twelfth Doctor into thinking the Doctor would try to circumvent a future where he appeared to die and become one of the ghosts, who blew up a dam and instead projected a hologram replica of his likeness as a ghost in the future. The Fisher King was caught in the flood and killed. (TV: Before the Flood)
Behind the scenes
The Fisher King shares name with a character from the Arthurian legends. The legendary king is wounded and waits for someone, who is able to heal him, to arrive. This is similar to the Doctor Who character who is waiting for someone from his people to come and save him.