Red Death

The Red Death was a biological weapon created by the Ice Warriors civilization.

It was a sentient poisonous mist that tracks a specific DNA pattern and hunts it down. This allowed them to programe the Red Death with the blood traces of an individual after which it was a simple matter of releasing the mist into the atmosphere of the targets planet. However, the mist was capable of reacting to the increased oxygen of a planet like Earth where it entered into a feeding frenzy making it target any living being as it slowly went beyond the Ice Warriors control. Such a weapon was quite capable of scouring a world of sentient life and turn a world like Earth into a plant more barren then Mars.

Lord Xznaal attempted to release the Red Death on Earth and planned for its use against the Doctor but was defeated by the Time Lord. (DW: The Dying Days)