The Cult of Skaro, named after the Dalek homeworld Skaro, was a secret group of four Daleks made up of three Dalek drones and one Supreme Dalek, The Cult of Skaro appear in six Doctor Who episodes called "Army of Ghost", "Doomsday", "Daleks in Manhattan", "Evolution of the Daleks" and the other two episodes called "The Stolen Earth" and "Journey's End", where only Dalek Caan appears since the other three were already dead.
Dalek Caan also appears in the two-part episodes "The End of Time" in Donna Noble's flashbacks.
The Cult of Skaro were at first just four ordinary Daleks. Dalek Sec was a commander of the Seventh Incursion Squad sent to wipe out the remnants of the Mechanoids which they succeeded in doing. Dalek Caan was an Attack Squad Leader of the Thirteenth Assault Group.
Dalek Thay was the commandant of Station Alpha, the most secret Dalek research facility. Dalek Jast was a Force Leader of the Outer Rim Defensive Battalion. After doing this all four of them were promoted to become the Cult of Skaro. They were given their names — making them some of the only Daleks with names — and Dalek Sec was chosen to be their leader.
The Cult of Skaro were assigned the task of furthering the Daleks cause through the development of new and unorthodox ideas and strategies. The leader of the Cult claimed that it was created primarily to further the process of Dalek evolution.