Dalek Sec in his armour, coloured black to signify his elevated position above the other members of the Cult.
"Our Creator... was wrong..."
-Dalek Sec-
Dalek Sec is a visionary. He began his career as an effective commander, leading Dalek forces to victory in many campaigns, notably the Dalek destruction of Installation 02. He also led the Dalek forces during the Dalek-Cyberman War, and was responsible for the verbal whipping of the Cyberleader during that campaign.
He lead a Dalek elite taskfore known only as the Cult of Skaro.
Dalek Sec after his transformation.
Despite being a Dalek he wanted to change how Daleks functioned, thus being the only Dalek to think so. He combined himself with a Human to become a Hybrid, and planned for all Daleks to do the same. However, Dalek Caan didn't much like the idea of getting legs so he took over control of the Cult, betraying Sec. Dalek Sec was 'accidently' killed by Dalek Thay later on, but really he got in front of Dalek Thay's gun to save the Doctor's life. Soon after, Sec's Dalek-Humans turned on their masters and destroyed Daleks Thay and Jast, forcing Caan to flee.
As a warrior Sec is shown to be quite competent, in his Dalek form that is. During the Dalek-Cyberman War he killed more than his fair share of Cybermen and even killed a few humans too. During the battle, the Cybermen allocated him as a primary target (along with the other members of the Cult). He also flew up into the sky and commanded the Daleks there, issuing the simple yet effective order 'Exterminate all life forms below!'. He was also the Dalek to kill the Monitor of Installation 02, 686 Pessimistic Insight, but in his timeline that was hundreds of years before the Hybrid incident despite the Battle of Installation 02 occuring hundred of years after the Hybrid incident.
In his human form, Sec is almost the complete opposite. despite initially wanting to convert all of humankind into Hybrids, his emotions later take effect and he comes up with a plan for the Daleks to escape to a world to live in peace. He sacrifices himself to save the Doctor, knowing he will make things right one way or the other. In the end, Dalek Sec gets the last laugh. Although his race of Hybrids are destroyed, Dalek Thay and Dalek Jast are annihilated fighting them and Dalek Caan is later driven insane, destroying the Daleks himself later in a mess of trickery, prophecy and idiotic ideas. Not only that, but the Humans survive, as does the Doctor. Dalek Sec: 1, the Universe: 0.
As Leader of the Daleks, Dalek Sec succeeds the Emperor of the Daleks and is succeeded by Dalek $wag, not unless you count Dalek Caan's brief control of the Cult. Technically, Davros is in there somewhere as well, but since he's not a Dalek he is not worthy of true leadership fo the Daleks.
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Doctor Who Daleks vs Cybermen