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 Legend) He was in command of the Seventh Incursion Squad, a squad of Daleks that chased The Doctor though time.
Your own leader! The only creature who might have led you out of the darkness, and you destroyed him!
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. Emerging, Sec served as Dalek commander during the Battle of Canary Wharf against the Cybus Cybermen and commanded the Daleks that emerged from the Genesis Ark.
Sec and the rest of the Cult of Skaro escaped a reversal of Torchwood's 'ghost shift' device via an Emergency Temporal Shift. (DW: Doomsday)
1930s New York
The Daleks must evolve. Evolve! EVOLVE!
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, however when that failed Sec decided to attempt the 'Final Experiment', blending Dalek with human. (DW: Daleks in Manhattan)
Human Hybrid
My Daleks, just understand this; if you choose death and destruction, then death and destruction will choose you.
While in his new body, he began to feel humanity for the first time in his existence, being motivated by emotions rather than the traditional Dalek drive to conquer. Motivated by these, Sec asked the Doctor to help him create more hybrids using human bodies, and started to express his feelings that their creator was wrong. He decided to have the new race live on, different from the Daleks, since if they continued to try to be superior, they would eventually die.
However, the other Daleks came to reject Sec's theories and eventually betrayed him while Dalek Caan took control and fueled the other human bodies with pure Dalek DNA. When the Daleks 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 up in the line of fire between himself and the Doctor, either saving the Doctor by sacrificing himself, or stood to halt the actions of Dalek Thay.
The Doctor showed his respect for Sec by 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 keep looking for death and destruction... then death and destruction will find you." Dalek Jast had dismissed the thought at the time, but a mere minute later he was the next of the Cult to die, by the hands of the Human Daleks. Dalek Thay was killed shortly thereafter. (DW: Evolution of the Daleks)
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