Phosphor Weapons (Gee try and guess what kind of chemical it fires...) are an ancient and very deadly form of unique weapon utilised by the armed forces of the Adeptus Mechanicus. If you haven't guessed by its name, Phosphor Weapons basically fire White Phosphorous in SPESS! Called Phosphex, though the crawling horror of Phosphex Weapons is all but forgotten in the Imperium, the blinding white fire of Phosphor weaponry is still utilised by the warriors of the Skitarii Legions to mark out high priority targets amongst battlefield murk.
If those struck by the burning spheres fired by these weapons are not killed by the initial trauma, they are soon felled by the light-guided volley that follows. Phosphor Weapons range from the hand-held Serpenta favoured by many Tech-priests, to the Heavy Phosphor Blaster mounted upon Onager Dunecrawlers. The burning white spheres that shoot out from Phosphor Weapons cling tenaciously to their targets, sizzling wildly as they melt into flesh or burrow through armour and exoskeleton alike. More often than not, it is these luminagenic orbs that deal the death blow. Even if a target proves tough enough to survive the burning pain, the phosphorescent glow these spheres exude can guide an ally's volley to strike the same target.
On the tabletop, generally speaking. Phosphor Weapons have the same strength and AP of a Heavy Bolter, but only single shot. Any wounds or hull points caused on a target will light them up like a christmas tree and reduce their cover save for the rest of the turn. Also makes them easier to charge at by letting you re-roll the charge range. Note: The phosphor pistol and serpenta hit harder and pierce armor better than bolters, in addition to lighting targets up. Remind me why the Imperium dropped these again. 8th edition Codex Adeptus Mechanicus says the Tech-Priest Veriliad destroyed the only STC for Phosphex weaponry, as he considered them too horrific to use. Naturally, they tied him to a stake and shot him to death with the best Phosphex gun they had, which is now a Relic, the Phosphoenix. This means that the Ad Mech is very reluctant to use their remaining stocks of Phosphex, making the weaker Phosphor weapons the main ones in 40k.
There are a few variants of these Phosphor Weapons.
Phosphor Blast Pistol
A pistol form of Phosphor Weapon that can be used as a sidearm by Skitarii Ranger/Vanguard Alphas, or Sicarian Ruststalker/Infiltrator Alphas, the Phosphor Blast Pistol is what you should get when you want to set someone alight but do no want to risk injury on yourself. On the tabletop, these are the Nerf Guns from hell as the pistol format Phosphor Weapon. It is shorter in range than the Serpenta, but you can take it in an infantry squad; no actual blasts, sadly. Design-wise, the Blast Pistol looks like a Pepperbox on hindsight. But upon closer inspection, you can clearly see that what you might have thought were barrels in a pack are actually rather long canisters – in essence, ammunition chambers. So by definition, the "pistol" is actually a very, very big revolver; presumably to be reloaded in a top-break manner, judging by the screw and the little handle attached next to the trigger guard. Of course, this handgun might really be a pistol if you consider that whole pack a one really, really big canister of Phosphex. Or if the canisters rotate at all.
A medium-ranged, pistol-type Phosphor Weapon used by the Sydonian Dragoons particularly to illuminate their would-be targets so that they can easily find and skewer them like shish kebabs, the Phosphor Serpenta is a more fancy looking weapon than the Blast Pistol in both aesthetics and function (And, I dare say... class?). It is so fancy that even a Magos Dominus – a Mechanicus equivalent to a general – can be expected to be holding in one of their many bionic hands. On tabletop, it is 18" Assault 1, essentially your squad's Markerlight when you want to follow up the attack with more shooting and probably a charge as well. Design-wise, in keeping with the steampunk-ish look of the Skitarii forces, it looks more or less like a 19th century revolver had a baby with a flintlock pistol. Considering the ornate theme, the Serpenta likely has a swing-out action which is no different from most of our modern revolvers, if it is a revolver. The center of the cylinder lies more or less at the same level to the barrel, making it more likely to be a single Phosphex canister instead of a conventional revolving cylinder.
A rapid firing Phosphor Weapon that is used as a secondary weapon by Kataphron Destroyers. On the tabletop, it is overall similar to that of the Phosphor Serpenta but it is now 24" and Rapid Fire. You know, just so you want to punish whoever is unfortunate enough to face it. Because the Serpenta version isn't already powerful enough and you just want to be sure that target is dead.
Heavy Phosphor Blaster
A large type of Phosphor Weapon that is deployed on Kastelan robots and in a twin mount on Onager Dunecrawlers. If you thought the regular Phosphor Blaster was bad, this thing takes the proverbial cake. On the tabletop it is a nice toy, 36" Heavy 3 S6 AP2 for fucking Guard over (wounding on 2s and ignoring saves completely), and it ignores cover. Probably the best Luminagen gun.