Dalek Sec was the leader of the Cult of Skaro in the British sci-fi series, Doctor Who. He could be distinguished from other Daleks by the all-black mechanical shell that encased his frail mutant form. Like all Daleks since the show's revival in 2005, Dalek Sec was voiced by Nicholas Briggs. When he was transformed into a Human-Dalek hybrid in "Daleks in Manhattan", he was portrayed by Eric Loren.
First appearing in the episodes Army of Ghosts and Doomsday, Sec led the Daleks into conflict with the Cybermen in The Battle of Canary Wharf. When the Doctor reversed the void-shift that allowed both armies to invade Earth, Sec and his fellow cultists escaped destruction by using an emergency temporal shift to jump back in time to 1930's New York at the height of the Great Depression. There, Sec and the Cult of Skaro set about the task of devising a new way to ensure the survival of the Dalek race.
In the episodes Daleks in Manhattan and Evolution of the Daleks, Dalek Sec determined that the Daleks must evolve and adapt if they were to continue. To this end, the Cult of Skaro built a genetic laboratory in New York's sewers, directly beneath the Empire State Building. They took control of the building's construction in preparation for their Final Experiment: a plan to combine human and Dalek genes, granting the Daleks' new bodies and removing the need for their robotic shells.
Dalek Sec summoned forth Mr. Diagoras, the man in charge of hiring staff to construct the Empire State Building, and decided to use him as a guinea pig in combining human and Dalek DNA. Sec was injected with a genetic solution and then opened his mechanical casing, pulling Diagoras inside with him. Inside the shell, Sec and Diagoras' forms began to merge into a single body, with Sec's consciousness remaining dominant but taking on certain psychological traits of the human Diagoras. Sec would later emerge from his shell, transformed into a Human-Dalek hybrid: a creature with a humanoid body but with a distinctly Dalek head, with a single eye, exposed brain matter and small tentacles protruding from either side of the head.
At the same time Sec emerged from his shell as a hybrid, the Time Lord known as the Doctor had arrived in the midst of the Cult's experiment. He interfered and helped several human captives escape, but Sec sent Dalek Jast and Dalek Caan after them. They emerged in the shanty town of Hooverville in Central Park, executing the community's leader Solomon as he pleaded with them to spare the residents' lives. After Sec witnessed this incident, he began to contemplate the Daleks' actions throughout their existence and determined that they needed to change their ways if they were to survive. He ordered Jast and Caan to return to the laboratory with the Doctor, believing that the future of the Daleks would depend upon him.
Dalek Sec convinced the Doctor to assist with the Final Experiment, but the other Cult members objected to Sec's new ideas. They relieved Sec of his authority and worked toward infusing their collection of human cadavers with Dalek DNA and Dalek ideals. They had arranged for the construction of a spire on top of the Empire State Building that would absorb the radiation of a solar flare, conducting the energy and animating the army of Dalek-Humans.
Dalek Caan took control of the Dalek-Humans and they were deployed against the Doctor and his friends inside an empty theater. Dalek Sec was chained to Dalek Thay's casing like a dog on a leash. He begged his fellow Cultists to let go of the Daleks' old ways of conquest and destruction, saying that it would one day be the end of them. The Daleks ignored him and Thay shot him dead.