The Cult of Skaro (named after Skaro, the original homeworld of the Daleks) was a secret group of four Daleks - three Standard Daleks and one Black Dalek - assigned the task of furthering the Dalek 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. Their willingness to adopt alien modes of thought was demonstrated by the fact that they had individual names, a concept most Daleks found abhorrent. Led by Sec, the Black Dalek, they answered to no one, not even the Dalek Emperor, although they still revered him.(DW: Doomsday)
History
The Cult of Skaro were at first just four Daleks. Dalek Sec was a commander of a Dalek group sent to wipe out the remnants of the Mechanoids which they succeeded in doing. After doing this all four of them were ordered by the Dalek Emperor to return to him. They arrived and were promoted to become the Cult of Skaro. They were then giving their names and Dalek Sec elected as their leader. (DWF: Birth of a Legend)
They were a secret order who were to think like their enemies in order to destroy them, although their original purpose was to imagine new ways of survival. During the Last Great Time War, they operated as a final strategic reserve - rather than fighting, they escaped into the space between universes, along with what they called the Genesis Ark, a piece of Time Lord technology. Having left the universe, avoided the destruction of the Dalek fleet.(DW: Army of Ghosts / Doomsday, Daleks in Manhattan)
The Torchwood Institute's meddling began to open the barrier to the Void which had been broke down by the Cult's Void ship. Torchwood Tower, known to the public as Canary Wharf, had been built around the gap into the Void so that Torchwood 1 could study it and use its energy. When the Void Ship opened, the Cult exited the ship and declared war - or "pest control" - against the Cybermen that had invaded Earth from Pete's World, thereby leading to the Battle of Canary Wharf.(DW: Doomsday)
The Cult escaped the battle through a temporal shift and ended up in New York City in 1930. They infiltrated the construction of the Empire State Building and began what they called the "Final Experiment" - the creation of a new Dalek race.(DW: Daleks in Manhattan / Evolution of the Daleks)
Inside the sewers below the Empire State building they set up a genetic laboratory and began trying to create new Dalek embryos. After this failed they tried thinking as the cult was designed to, and came up with the idea that they should use earth's greatest resource, its people. They placed Dalekanium upon the mast of the Empire State building and devised a plan to fuse the DNA of Daleks and Humans using an incoming ray of gamma radiation from the sun.
Dalek Sec tested this on Mr Diagoras, he became a Dalek-Human hybrid, now with human thoughts and emotions. Dalek Sec wanted to make the Dalek-Humans' DNA 90% human. The other Daleks, although bred to obey Sec, knew it was against the Dalek cause and so changed the DNA to 100% Dalek and imprisoned Dalek Sec and later exterminated him accidentally after they took a shot at the The Doctor
As the gamma radiation came in the Doctor got in the way of the blast and so confused the DNA with some of a Time Lords. Therefore the Dalek-Humans rebelled and killed Dalek Thay and Dalek Jast. The Doctor offered Dalek Caan - the last surviving Dalek - help but instead of taking it, he used an emergency temporal shift to teleport out.
- See the individual sections on members for what is known about their respective histories:
Members of the Cult of Skaro
Dalek Sec
Dalek Sec was at first the Commander of a Dalek force whose mission was to wipe out the Mechanoids. Sec commanded from on the battlefield overlooking the field from vantage points. After the Mechanoids were Exterminated Sec returned as ordered to the Emperor. There Sec was selected as the leader of the Cult and given his name he was then sent to the weopens research area for upgrades. He was newly housed in Metalert Dalekanium that was more defensive than the average Dalekanium. Sec along with the rest of the Cult then went on to begin using their initiative to defeat enemies. (DWF: Birth of a Legend)
Dalek Sec was responsible for the Battle of Canary Wharf in 2007. He was designated by his black casing (metalert-enhanced). (DW: Doomsday)
In "Doomsday" Dalek Sec was forced to use emergency temporal shift to avoid being pulled into the void. This teleported him and the rest of the cult to 1930's New York.
Sec was notable for apparently possessing telepathic abilities in addition to the telekinetic power common to all Daleks. He displayed a capacity for truly radical thought, being the first Dalek to believe that their physical form could be in any way less than perfect, an idea which even the other members of the Cult found initially repellent. As a result, he became the first Human-Dalek hybrid as part of the "Final Experiment", combining with Mr. Diagoras, using his flesh to become independent of the Dalek shell. However, the other Daleks eventually relieved him of his command, no longer considering him to be a Dalek. He was exterminated by Dalek Thay after taking a shot meant for the Doctor. (DW: Daleks in Manhattan / Evolution of the Daleks)
Dalek Jast
Former Force Leader of the Outer Rim Defensive Battalion, Dalek Jast was a member of the Cult of Skaro and a participant in the Battle of Canary Wharf in 2007. Jast was the first to realise that the Doctor was still alive after the arrival of the Cybermen when he noticed the Doctor on a video transmission with the Cyber-Leader. He had the highest-pitched voice out of the members of the Cult.(DW: Doomsday)
Like Caan and Thay, he was created to follow Sec but had doubts about his leader when Sec became the first Human-Dalek. Along with Caan and Thay, Jast turned against Sec. When the Dalek-Humans questioned their orders after Gallifreyan DNA was mixed with theirs, the new Daleks battled Jast and Thay, resulting in Jast's destruction. (DW: Evolution of the Daleks)
Dalek Caan
Former Attack Squad Leader of the 30th Dalek Assault Squad,Dalek Caan was present at the Battle of Canary Wharf in 2007 and was possibly the second-in-command after Sec. He had the lowest-pitched voice out of the members of the Cult. (DW: Doomsday)
He was the Dalek Mr Diagoras summoned and spoke to when needed in 1930 New York City and served as the Cult's link to the outside world. (DW: Daleks in Manhattan)
After Sec became a Human-Dalek, Caan and the other Cult members no longer considered him to be a Dalek and turned against him. Caan made himself Controller and became linked to the Dalek-Human army, but when the Dalek-Humans rebelled and destroyed Jast and Thay, he destroyed them in turn. Now being the last surviving Dalek in the universe, the Doctor confronted Caan but the Dalek escaped from him via an emergency temporal shift. (DW: Evolution of the Daleks) it is rumoured that caan will return along with davros at the end of series four, along with a new dalek army made by caan.
Dalek Thay
Former Commandant of Station Alpha, Dalek Thay was a member of the Cult of Skaro and was the first Dalek to encounter the Cybermen during the Battle of Canary Wharf in 2007. Thay was tricked into identifying himself by the Cybermen after both sides refused to "identify first". Thay spoke in a low-pitched voice. (DW: Doomsday)
In 1930 in New York City, Thay originally had strong feelings against the Final Experiment, claiming that it was against the Dalek ideology of racial purity, but eventually conceded that it was necessary for survival. To complete the Empire State Building before the gamma strike would hit Earth, four of the back plates of Thay's casing were removed and attached to the building's mast, being replaced with iron as the bonded polycarbide of the plates was needed to attract the gamma strike. (DW: Daleks in Manhattan)
The Dalek-Humans, after Gallifreyan DNA was mixed with their Dalek and Human DNA in the "Final Experiment", questioned orders and revolted against their Dalek masters, destroying Thay and Jast. (DW: Evolution of the Daleks)