A Phosphex Bomb and its artillery counterpart, the Medusa Shell, are arcane incendiary weapons utilised by the military forces of the burgeoning Imperium of Man during the Great Crusade and Horus Heresy eras in the late 30th and early 31st Millennia.
Phosphex was a rare corrosive toxic and incendiary compound utterly inimical to life, deployed both in the form of large canister bombs (called Phosphex Bombs) and heavy artillery shells (the horrific Medusa Shell). It expands on contact with air into a seething, liquid mist which burns with an eerie white-green flame which is attracted to movement. This gelid flame ignites metal and eats relentlessly into living tissue, and cannot be extinguished short of exposure to vacuum. Though highly effective, this horrific weapon was only primarily used as a weapon of last resort.
As effective as Phosphex proved on the battlefield, its use was not widespread as it had a tainting effect beyond even that of the Rad Weapons utilised by Space Marine Legion Destroyer Squads on the areas in which it was employed. However, Phosphex Bombs and Medusa Shells remained within the arsenals of the Legiones Astartes as a weapon of dire resort. Phosphex is no longer in use by the Imperium in any large scale capacity.
- The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pg. 231