These are the Salamanders vision for a better, more ethical, legion destroyer squad. In some ways a little bit like the Word Bearers, their thinking here is to sear the world in purifying flame. But this is not being driven by any religious viewpoint. They are genuinely purifying and giving the potential for rebirth.
As a squad, they're equipped with a special type of flamer, the Pyroclast flame projector. It can fire in two modes: both a template and a six inch range pseudo-melta mode. The problem with the latter is the reduced strength (even accounting for the effect of the Promethean Gift rule). So its only going to be good for transports like rhinos with any certainty - forget taking on land raiders. Take melta bombs if you want to go this way.
In addition to this, they all have the mantle of ash armour. Giving a 2+ save, this kind of equipment in 30k is amazing for an entire unit outside of terminators. But on top of this is an invulnerable save which is only good against flamers, plasma, volkites and melta type weapons. Hence in many ways, the best way to think of the pyroclast squad is like a terminator unit. Sure, they can get gunned down by mass bolter rounds, just like terminators, but they're cheaper but a reasonable margin. For this reason, they are a very tempting addition to Heavy Support selections for the Salamander. If only we could teleport them on to the battlefield, it'd make such a huge difference. That said, they can take land raiders as an option, so I think that'd be compulsory outside of a descent of angels style drop assault detachment.
Here are a few build ideas.
5 Pyroclasts, the Pyroclast Warden with a Powerfist. (175 points).
I think this is a baseline squad. Given that the Salamanders special rules make them unlikely to break due to fear, they're going to hang around significantly. Use with a drop pod assault rite of war if possible. OR: Pile them in to a land raider, and you've got a good unit that can cause serious damage to enemies in cover, or just about anything short of a land raider itself. And for that, one can always upgrade to:
5 Pyroclasts, all with meltabombs, Pyroclast Warden with a Powerfist. (200 points).
Now you're talking dangerous to almost everything in the game short of Lords of War. Place in a land raider and enjoy yourself.
10 Pyroclasts, all with meltabombs, Pyroclast Warden with a Powerfist. (325 points).
The full version of the above. This is a great unit that benefits from the unit-upgrade cost of the melta bombs. Save points here, but remember to use them wisely.