Dalek Sec

Dalek Sec was the Commander of a Dalek force whose mission was to wipe out the Mechanoids. Sec commanded from the battlefield, overlooking the field from vantage points, and finally exterminated the Dalek's old enemies. (DWF: Birth of a Legend) He was in command of the Seventh Incursion Squad, a squad of Daleks that chased the Doctor though time.

Following these missions, he served as leader of the Cult of Skaro. They escaped the Last Great Time War by hiding between realities in a Void ship.

The Battle of Canary Wharf
Emerging from the Void Ship with the Genesis Ark, a Time Lord device, Sec served as Dalek commander during the Battle of Canary Wharf against the Cybus Cybermen. He and the other Daleks seemed unhindered by the Cybermen's invasion, stating 'This is not war - this is pest control!' Sec led the Daleks to the top floor of Canary Wharf (Torchwood Tower) where the Cult of Skaro opened the roof and and ascended to an area where the Genesis Ark could open.

Sec and the rest of the Cult of Skaro released an army of Daleks from the Genesis Ark, which started to kill the Humans and the Cybermen who were also present at the battle and in the streets below. If it were not for the Tenth Doctor's involvment opening the void, he and his army would likely have won the battle as Daleks could easily kill Cybermen while Cybermen had a lot more difficulty killing Daleks (indeed it took a large force of them combined with humans equipped with alien weapons to damage a Dalek's shielding). Instead, Sec and the Cult initiated an emergency temporal shift and vanished before they were flung into the void.(DW: Doomsday)

1930s New York
"The Daleks must evolve. Evolve! EVOLVE!"

- Dalek Sec.

Sec along with the Cult of Skaro ended up in New York in the 1930s and established a base beneath the Empire State Building. The Cult of Skaro attempted to create new Dalek mutants and revive the Dalek race; when those attempts failed, Sec decided to attempt the "Final Experiment": blending Dalek with Human. He enveloped Mr Diagoras and began to "evolve". (DW: Daleks in Manhattan)

Human Hybrid
"My Daleks, just understand this; if you choose death and destruction, then death and destruction will choose you."

- Dalek Sec.

While in his new body, he began to feel humanity for the first time in his existence, Motivated by emotions beyond anger and hate, and rejecting the traditional Dalek drive to conquer, Sec asked the Doctor to help him create more hybrids using mindless human bodies. Sec started to express his feelings that their creator had been wrong. He decided to create the new race of Dalek Humans, different from the Daleks, out of the belief that if they continued to try to be superior, they would eventually die.

However, the other Daleks came to reject Sec's theories as heresy, and eventually betrayed him. Dalek Caan took control and fueled the other human bodies with pure Dalek DNA, in place of Sec's distinct mixture of the two. When the new Daleks in human shape tracked the Doctor down, they took Sec with them, putting him in chains. Taken before the Doctor, Dalek Thay accidentally exterminated Sec when he stood between Thay and the Doctor, either intending to save the Doctor by sacrificing himself, or expecting Dalek Thay to obey him and stop the mutiny.

The Doctor openly declared his great respect for Sec, calling him the "cleverest Dalek ever" and the only creature that could have led the Daleks from the darkness.

Dalek Sec made a prediction mere seconds before his death, stating "My Daleks... understand this. If you choose death and destruction... then death and destruction will choose you." Dalek Jast dismissed the thought at the time, but a mere minute later he was the next of the Cult to die, at the hands of the Human Daleks. Dalek Thay was killed shortly thereafter, leaving only Caan, who performed an emergency temporal shift and disappeared. Exactly what happened to the casing he left behind, or indeed his body and that of his killers is unknown. (DW: Evolution of the Daleks)