The Cult of Skaro
The Cult of Skaro first appeared in Army of Ghosts (Series 2 : Episode 12) and Doomsday (Series 2 : Episode 13). The Cult of Skaro consisted of four members, Dalek Sec (leader), Dalek Caan (second in command), Dalek Thay and Dalek Jast. Together, these four Daleks were tasked with ensuring the Daleks survival during the Time War. They were chosen by the Emperor of the Daleks, though they were above the Emperor in terms of authority. They were made to think like their enemies and find new ways of surviving. This led to them having their own imaginations. When we first saw the Cult of Skaro, they believe themselves to be the only four Daleks in existence, due to the demise of the Emperor in The Parting of the Ways (Series 1 : Episode 13). They survived the Time War by hiding in the Void between worlds in a Void Ship along with a Time Lord prison called the Genesis Ark. The Genesis Ark contained imprisoned Daleks, which were released when Mickey Smith accidently touched the Ark, triggering its release process.
When the Doctor reversed the radiation flow from the Void to suck the Daleks back in, the Cult of Skaro performed an “Emergency Temporal Shift” to escape. This appears to be an action that can only be performed by the Cult of Skaro as no other Dalek has used it. Using this temporal shift, the Cult of Skaro landed in 1930’s Manhattan (Daleks in Manhattan (Series 3 : Episode 4)), where they attempted to further the evolution of the Daleks. Underneath the Empire State Building the Daleks planned to turn humans into Dalek Hybrids. Dalek Sec was the first to test this experiment by merging with a human. The experiment was a success as far as Dalek Sec was concerned; however the others were not as happy. Dalek Sec gained some humanity in his conversion, and realised that the Dalek Hybrids also needed their human emotions to evolve and survive properly, stating that humans had survived for centuries while the Daleks were always close to extinction.
The other members of the Cult of Skaro disagreed with this theory and injected 100% pure Dalek DNA into the hybrids and mutinied Dalek Sec. Dalek Caan took leadership of the Cult during the final stages of their plans. Though Dalek Sec continued to try and reason with his fellows, they refused to listen to him as he had been corrupted. When Dalek Thay went to exterminate the Doctor, Dalek Sec took the blast for him, which the Doctor honoured by saying that Dalek Sec was the “cleverest Dalek ever” for realising what it was the Daleks needed to do to survive. Dalek Caan then tried to get the Dalek Hybrids to kill the Doctor, however because he had intervened with the DNA input by adding a bit of his own with the Dalek DNA (using a technique called splicing), the Hybrids turned on their creators and killed Dalek Thay and Dalek Jast.
Dalek Caan then chose to kill the Hybrids because they had been tampered with rather than letting them live. By the end of this encounter with the Cult of Skaro, Dalek Caan was the only member left alive. Dalek Caan performed another “Emergency Temporal Shift” and somehow ended up in the time-locked Time War. Here he rescued the Daleks creator, Davros, and set about creating a new race of Daleks. The journey into the Time War however destroyed Dalek Caan, rendering him useless other than his new prophetic abilities. However, these new abilities allowed him to also see the whole of time and he saw the destruction the Daleks caused throughout history and what they would yet do and set about a plan to draw the Doctor to the Crucible in The Stolen Earth (Series 4 : Episode 12) who could stop the Daleks reality bomb. When the Crucible was destroyed it is believed that Dalek Caan died as well, thus ending the Cult of Skaro.