Photon Thruster Weapon
Despite the name, Photon Thruster weapons are super weird weapons designed by the Adeptus Mechanicus that has very little relations to light weaponry. In a nutshell, they fire black laser fire. Photon Thruster Weapons are advanced weapons by the Myrmidon Destructors and Thallax of the Adeptus Mechanicus. The arcane secrets of these deadly beam weapons are closely guarded by the Tech Priests of the Mechanicum. A possible explanation of this 'black fire' or 'black light' is that it could be a weaponized form of dark matter. Or maybe accelerates photons beyond human perception, giving the impression of black coloring. Just like Plasma Weapons. They are subject to the 'Gets Hot' rule.
Coincidentally, this is a similar property to the Dark Eldar's Blast Weapons which shoots a similar black laser affect.
Photon Gauntlet
The pistol version. Only available to Magos Primes, Magos Dominus and the Secutarii Axiarchs. As the pistol of the family, its range and strength is far shorter than its larger brothers, with a max range of 12" and S 5, it is overall only suitable when carrying a much larger CQC weapon on another hand.
It's armor piercing is decent but the chance of overheating is twice that of a Plasma pistol. Nor is it a 'Lance' weapon like it's larger cousins. Making it overshadowed by less risky Archaeotech pistols and Machinator Arrays. The latter coming stranded with a more devastating Flamer. All in all, the Photon Gauntlet sits in a very awkward position, if you want good anti-armor than a Melta is more feasible due to its longer range, if you want good anti-infantry than a Flamer could cover more ground at roughly the same range. They can't all be winners.
Photon Thruster
The mainstay Photon weapon. The Photon Thruster is one of the primary weapons of the Thallax. When fired, it unleashes a howling needle-thin beam of blackness able to pierce through the densest matter and easily able to rip apart the most heavily armored men and machines apart. The power source of these weapons are extremely unstable little understood even by the Myrmidons themselves, and a catastrophic failure can lead to its wielder being consumed by raging black flames.
Of course, the Thallax are considered somewhat disposable and that any accidents could easily be replaced.....except for the ludicrously expensive and hard to maintain Photon Thrusters.....bummer....
Darkfire Cannon
The largest of the Photon Thruster weapons.
Believed to be of alien origin (though nothing has ever been proved, granted, it could be of Dark Eldar origin as mentioned above), the Darkfire Cannon is a long lance-like weapon that fires needle-thin beams of dark energy that can pierce even the thickest armor with ease. Unleashing screaming beams of pure darkness, armour, flesh and stone boil away with their touch, and even the most heavily armoured vehicles can be torn apart by their hellish un-light.
Only the Castellax Battle-automata and above is big and powerful enough to carry these.