Pyronite is the name of a substance combined with the radioactive crystal known as Kryptonite and the element of fire. It is the very lethal weaknesses to the variant of Tri-Breed known as The Progenitors.
Pyronite wasn't created until the hero known as Hybrid started showing up. Lex Luthor realized that with Hybrid in the picture, he may pose a great threat to his corporate empire. This made him send out several supervillians to face the hero. The first of which was Metallo with a Kryptonite heart, however it proved ineffective against Hybrid. He also sent Firefly to assist Metallo and that's when both exposed him to both Kryptonite and Fire.
Seeing that combined attacks with both substances caused him great pain, Lex realized what he needed to take him down. LexCorp then worked and succeeded on fusing both Kryptonite and fire. Naming it Pyronite, Luthor decided to use it against Hybrid. In their first encounter, Hybrid proved to have become very weakened by the substance at close and moderate proximities.
Once away from the substance, Hybrid regained his strength and eventually was exposed to the substance once more. This time Hybrid showed a form of resistance to the substance. After at least a minute of resistance time, Hybrid's vulnerability to the deadly substance returned and his strength diminished greatly. This was until Superman flew into play and took the lethal substance away from Hybrid and took down Luthor.
Pyronite is known to specifically target Tri-Breed variation of The Progenitors. The reason for this substance being lethal to them was the fact that their physiology is composed of Kryptonian (Susceptible to Kryptonite) and Green Martian (Susceptible to fire). Surprisingly though because the Kryptonite radiation neutralizes the effects of fire, the substance has no effect on a pure Green Martian and because of the fire's neutralizing effect over the Kryptonite's radiation, Kryptonians are not affected.
The substance is shown to harm Tri-Breed beings at close to moderate proximity and causes drainage of their natural physical power. Prolonged exposure to the substance will cause them to die, however it also has a benefit. The benefit is the amount of time one has in account of temporary natural resistance to Pyronite. The substance however can be blocked indefinitely by a sub-form of Lead known as Aqueous Lead. This strange lead will definitely neutralize both the harmful effects of both the Kryptonite radiation and the radiation given off by the fire.