Promethium

Promethium is the 40K equivilent of napalm, and is used in flamers to provide a lethal stream of fire. If properly blessed, it can be used for incinerators, and thus can be used by the Ordo Malleus and Hereticus to purge their enemies. It is also the standard military fuel, used for tanks and planes. Promethium can burn in space and underwater, making it extremely useful for boarding actions where the enemy might not expect to see flames in an airless environment.