Dalek Sec was the leader of the Cult of Skaro. The Cult of Skaro were formed to find new ways of fighting and defeating the enemy. They survived the Time War by hiding in a Void Ship which was placed outside of time and space.
This Dalek appeared in the TV stories Army of Ghosts and Daleks in Manhattan.
Pictures above are from the TV story titled Army of Ghosts
Pictures above are from the TV story titled Daleks in Manhattan
This is not war. This is pest control!