The entity crashed to Earth in the Weserbergland Mountains in Lower Saxony in 1283, and by the following year had broken loose from the meteorite by feeding on the local inhabitants' fear of the plague. Its first major act was to take the children from the city of Hamelin in order to elicit fear from their parents; an act that made it known as the Pied Piper.
The Pied Piper took the children to its own realm, which included such wonders as toffee apple trees and lemonade fountains, but they soon tired of these and began longing for their parents. The Doctor, John and Gillian arrived and decided to help free the children. They traveled to the Pied Piper's castle, who challenged the Doctor to pass three tests or become his prisoner. The Doctor used various technological tricks to pass the tests, then led the children back to Hamelin.
Over the next seven hundred years, he continued to abduct children in order to create fear. One notable stint was an eight year period between 1932 and 1940, in America, as "Odd Bob the Clown". He was also responsible for instilling coulrophobia in young Sarah Jane Smith.
In 2009, he set up Spellman's Magical Museum of the Circus in London in order to steal children from the city. This was made possible by the fact that the Weserbergland Meteorite was on loan to the nearby Pharos Institute. The now adult Sarah Jane Smith prevented him from stealing the children, but he abducted Luke Smith and imprisoned him in his hall of mirrors. With the help of Rani Chandra and Clyde Langer, Sarah Jane overcame her fears, which weakened him to the point where she could force him back into a fragment of the Weserbergland Meteorite. She then placed the fragment in an alien box from which nothing could escape.
The pied piper is a very powerful entity, its powers include (but are not limited to) Teleportation, Telekinesis, Telepathy and Mind control (although the victim needed to touch or be holding something for him to channel it into them)and Shape Shifting. He needs fear to power his powers.
His shape shifting is the most seen of his powers as several times through out the story he transformed his body clothes and voice to a new form, he however only shows three forms in total so it is unknown if he can have more or though its hinted he can.