Dalek Sec was the leader of the Cult of Skaro, a group of four Daleks from their homeworld of Skaro. Sec was the former Commander of a Dalek force whose mission was to wipe out the Mechanoids. Sec commanded from on the battlefield overseeing the field from vantage points. After the Mechanoids were exterminated, Sec returned as ordered to the Emperor. Sec was selected as the leader of the Cult and given his name. He was then sent to the weapons research area for upgrades, where he was re-encased in Metalert, a type of Dalekanium that was defensively superior to standard Dalekanium.
Sec, along with the rest of the Cult, began using their initiative to defeat enemies., untill the Emperor promoted him and three other Daleks, they then became known as the Cult of Skaro above and beyond the Emperor himself. Their objective was to discover new ways of survival and to think as the enemy thought. Sec was then given a new Black Dalek casing, different than the normal standard gold casing.
As the Time War started, Sec and the Cult fled the war in a void ship in order that the Dalek race would survive, they took with them a Genises ark, a prison ship, small on the outside but massive in the inside similar to that of a Tardis. Soon in 2007 Earth, the Torchwood institute had found the void ship and attempted to open it and use its power for mankind's fight against aliens.
Shortly after the sphere opened and Sec led the cult out and encounter three humans and prepare to exterminiate them. Just before Sec can kill them, a female identifies them as Daleks and tells them she also knows about the Time War, Sec and the others stand shocked and motionless at this, Sec decides to keep them alive so they could be used. Sec asks Dalek Thay about the ark, Thay replies it is in hibernation. Sec and the Cult prepare the ark to be opened. However the Cult soon realize that another force is at work and are already invading Earth, the Cybermen, 5 million Cybermen worldwide, the Daleks ignore this and continue work on the ark untill the female reveals that their greatest enemy the Doctor is there, Sec and his Daleks stand shocked and afraid of this news, but continue the work untill the Cybermen burst in the room and attack! Sec and his Daleks are more than a match for the Cybermen and easily kill them, Sec continues the battle and manages to open up the ark unleashing millions of Daleks, luckliy the Doctor opened the void which sucks the Daleks and Cybermen back in, but Sec and the Cult uses a emergency temporal shift to escape. Sec and his three Daleks Thay, Jast and Cann are now the only four Daleks in existence.Sec and the Cult arrived in 1930's Manhatten New York, they make their base under the Empire state building and create a labratory and conduct experiments to rebuild the Dalek race, however the experiments failed and they leave their failed Dalek embryo's in the sewers to die. The Cult then kidnapped humans and transformed the dim ones into Pig slaves while the smarter ones were kept, Sec then took a human called Mr. Diagoras and made him merge into him, thus creating the first Human Dalek. Sec soon emerges from his casing, now half human and half Dalek, he begins to feel humanity for the first time a Dalek has ever felt for thousands of years ever since Davros their creator altered them. Soon they encounter the Doctor again, Sec needs help from the Doctor to create more Human Daleks, this would give the Daleks a new chance to start again without hatred. The other Cult members are outraged by this, the Doctor agree's to help since Sec want to end the Dalek's evil nature, sadly the rest of the Cult are not happy about this and they soon betray him and make Cann the new leader. In the end Sec was exterminated by his own Daleks while trying to stop them from killing the Doctor. The newly created Human Daleks soon kill Jast and Thay before Cann kills them and escapes by using Emergency temporal shift. The Doctor despite hating the Daleks had respect for Sec, because he could have been the only Dalek that could of brought the Daleks out of the darkness.