A highly flammable and illegal substance
Appearing as a clear, viscous material, Phos-Ex is actually a colony of microbial life forms, artificially created. They act as enzyme disintegrating metal and devouring flesh, the reaction causing extreme heat and fire. Even contact with open air causes a fire to erupt from it and the substance does not stop until either it or what ever it consumes has been thoroughly destroyed.
It is illegal because mis-use of it can cause a fire that will eventually grow to cover an entire planet. It is due to this that it is banned by nearly ever single civilization that knows of it.
The only known thing that holds it back is a lack flammable gases, such as the void of space, or the canisters that hold it.
The Planet Myria, in the territory of the Cross System Empire, became known as the burning planet when the empire used the experimental weapon. Afterwards the substance was declared illegal as the other star empires threatened to unite against the Cross System Empire, due to the substance leaving the world useless.
It would eventually be used to create Ignisium