Sec as a human-Dalek was like a cross between a Dalek and a human form, with a Kaled mutant-shaped head (including six tentacles on the sides and with a partially-exposed brain), and brown, slimy skin. Other than that, he was identical to a human in all respects. He was also prone to feeling human emotions. When Sec absorbed the human body, he remained in his armour until the assimilation was complete.
The human-Daleks originated out of the many experiments on Dalek flesh in the sewers of New York City in 1930. These experiments, research of the Cult of Skaro (who had temporally shifted from the Battle of Canary Wharf), resulted in the so-called "Final Experiment" after experiments to grow new Dalek embryos failed.
This "Final Experiment" involved Mr Diagoras, head of constructing the Empire State Building and the human selected as most Dalek-like, being merged with the body of Dalek Sec, leader of the Cult, inside the Black Dalek's casing. The experiment was to allow the Daleks to have a new life and to recreate the Dalek Empire. (TV: Daleks in Manhattan)
The Dalek-human-Time Lord army
Later, the Daleks, under command of Sec, set to work combining the bodies of several hundred humans who had been placed into stasis with a mixture of Dalek and human DNA. However, Sec's subordinates sabotaged the project, causing fully-Dalek DNA to enter the minds of the humans, instead of the hybrid forms.
However, the Tenth Doctor sabotaged this work by attempting to destroy the energy conductor at the top of the Empire State Building, and when this failed clung to the conductor as a solar flare lightning strike hit the conductor, causing the Doctor's Time Lord DNA coding to be mixed with the Daleks', creating Dalek-human-Time Lords who disobeyed the Daleks' orders (to the Daleks' surprise) and destroyed Daleks Thay and Jast. (TV: Evolution of the Daleks)
Destruction of the Dalek-humans
Sec as a human-Dalek was the first to be destroyed, by Dalek Thay, standing in front of the Dalek's gunstick which was aimed at the Doctor.
The Dalek-humans were killed while they were revolting by Dalek Caan, who sent a fatally high-pitched sound to their ears, ending their species while Caan deemed them a failure. (TV: Evolution of the Daleks)