The death whip was a weapon in possession of Fadros Pallujikaa, leader of the Sycorax.
Properties
Composed of a brown rope and a handle housing the micro-fusion generator, the rope was pulled back and hurled at a human's neck, ensuring a prompt death. During this process, the rope would illuminate blue and show the victim's bones in the same colour, much like a Dalek's death ray. This destroyed all cells in a human body as well as disintegrating clothes, leaving only scorched bones behind. (TV: The Christmas Invasion, PROSE: The Final Darkness)
History
During the Sycorax invasion of Earth on Christmas 2006, Pallujikaa used his death whip to murder Daniel Llewellyn and Blake, who were part of a delegation of humans he had teleported aboard his ship. He endeavoured to reproduce the results on the newly-regenerated Tenth Doctor after he emerged from his TARDIS, only for the the latter to grab the whip a split-second before it came into contact with his neck, force it from his grip and snap it, discharging the energy within it. (TV: The Christmas Invasion, PROSE: The Final Darkness)
Behind the scenes
The term "death whip" is used by a Sycorax scribe in The Final Darkness. "Sycorax whip" is used on merchandise such as Battles in Time.