Photon Thruster Weapon
Despite the name, Photon Thruster weapons are super weird weapons designed by the Adeptus Mechanicus that has very little relation to light weaponry. Supposedly, they fire "black laser fire" (not uv, not purple, black). Photon Thruster weapons are wielded 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. Just like Plasma Weapons, they are subject to the 'Gets Hot' rule. On the other hand, perhaps photon thruster is because whatever it is firing forces photons away. Hence, black.
The shotgun version. Only available to Magos Primes, Magos Dominus and the Secutarii Axiarchs. As the baby of the family, its range and strength is far shorter than its larger brothers, with a max range of 12" and S 5, though like them it does at least fire twice (being Assault 2 instead of Heavy 2).
Its armor piercing is decent but the chance of overheating is twice that of a Plasma pistol. It isn't a 'Lance' weapon like it's larger cousins, making it overshadowed by less risky Archaeotech pistols and Machinator Arrays (the latter coming standard with a more devastating Inferno Pistol/Flamer combo). All in all, the Photon Gauntlet sits in a very awkward position of being nothing spectacular, 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. About the only time it's a decent choice is with the Secutarii Axiarch who gets it for dirt cheap and can't have any other AP2 weapons (or a flamer). The fact that it isn't a pistol also doesn't come into play if he takes a power fist.
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....
On the Horus Heresy tabletop, the Photon Thruster is a 48" S6 Heavy 2, Lance weapon that hits at AP2 whilst causing Blind. Like all Photon Thruster Weapons, it has the Gets Hot! rule, but you are able to penetrate the rear end of Terminator Armour like Swiss cheese.
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 or larger are sturdy and powerful enough to use these.
On the Horus Heresy tabletop, the Darkfire Cannon are a strong choice, it's better than the Melta's at taking down 2+ armour (such as terminators) and its long-range can help lay down a lot of hurt on other Monstrous Creatures or Primarchs. It also forces units to take blind tests which could really tip the balance of power when it comes to assault them (remember; monstrous creature, you have relentless). Ignore the lance factor as you're still glancing on 5's.