The Druid was a renegade Time Lord from the planet Gallifrey.
He time travelled to Earth during the events of the last great Time War.
He regenerated into Amelia Airhart because he was wanted and people were looking for him.
One day the Druid was injured in a plane crash so then he regenerated into a man again. He took the disguise name Rupert Drake and was a gang leader for 4 years and then he died in a fight against mobsters.
He regenerated again and went back to being called the Druid. He created a monster called the Killer by firing his thunder screwdriver. The Killer was hired to kill the Doctor but eventually in a battle with the Doctor and Lucie, the Killer died. The Druid confronted the Doctor and Lucie four times.
He was burned bad and died in a building when there was a fire so he regenerated into a new incarnation and then time traveled to ancient Greece and he pretended to be Hades. But the Doctor and Donna figured out he was really the Druid. The Druid was later revealed to be the Doctor's uncle Farajo Sigma.
He then tried to take over the world. He was killed by an angry mob and died without regenerating.
A while later his soul went into the body of a robot and adopted the name the serial killer. His current whereabouts are unknown but some say that he is living in an old abandoned mansion in Transylvania. He hired Abzorbaloff to assassinate the Doctor but he failed.