The Red Death was a biological weapon created by the Ice Warriors.
It was a sentient poisonous mist that tracks a specific DNA pattern and hunts it down. The Red Death was created through a technique that combined molecular re-engineering and artificial intelligence etherware; the latter of which provided it with an animal intellect. 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. Typically, once it had performed its function, the mist would dissipate. 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.