Destroyer Squad

An elite formation of the Space Marine Legions of old, Destroyer Squads are the guys tasked with doing the dirtiest work other Space Marines would shun. They were formed to fight in a very destructive way, so what was the problem? They did so by using rad and toxic weaponry, along with such nasty gear like the now lost Phosphorex weapons (bombs in this case). Stuff that was essentially morally questionable and dishonorable in general from the P.O.V. of even the most pragmatic Legions (and that's a lot).

The origins of this unit came during the Unification Wars of all things, as they were inspired by the various enemies The Emperor faced when he was conquering Terra in preparation for the Great Crusade. Despite the destructive potential, the Legiones Astartes shunned them, seeing in the Marines that were part of Destroyer Squads as the equivalents to That Guy (as in the marines themselves were morally dubious for even willingly joining those formations). Nevertheless, Destroyers were seen as a necessary evil by Imperial Commanders for being so damn efficient, though most of the Legions either shunned them, as mentioned previously, or simply didn't have them at all.

Due to the loadout that would make Papa Nurgle happy, the Legionnaires who were part of the Destroyer corps were heavily affected by their own weapons. Marines would have as side-effects abnormally pale skin, said skin rotting away due to cellular degeneration, tainted veins and blood bruises on the flesh, yellow eyes, and finally various aggressive forms of cancer that were so bad that required cybernetic replacements.

Needless to say, the one Legion that would be associated the most with these guys would be the Death Guard. Having their super-human resistances further boosted with capabilities of Mortarion's geneseed, the Death Guard would become the experts on biological warfare and send to places that would prove fatal for normal Space Marines. And because of their love for this type of warfare, the already dubious Legion would be seen with even greater suspicion by the rest of the Legio Astartes for even emitting an aura of taint and horror. A prelude to what the Legion would turn into in the future.

The Dark Angels had their own formation built around Destroyers called the Dreadwing. Armed with the best mobile armor and the most devastating of weapons. Such as Phosphex and Vortex weapons. Allowing them to use Fellgraves equipped with Warp Cannons and Vindicators firing Vortex Shells. The really nasty shit they deploy to get rid of a target the commander in charge wants gone immediately.

Pyroclasts

A specialist unit based on the Destroyers and unique to the Salamanders Legion, Pyroclasts are pyromaniacs even by the Salamander's fire fetish standards. Vulkan saw little place in his Legion for Destroyers (due to their status as That Guy), so he reduced their number to a required minimum he used for xeno-purging, whereas the now naked majority of them he armed with Pyroclast Flaming Projectors. Weapons they were named after and which Vulkan himself designed.

The weapons of these warriors were essentially equivalents to Fusion Projectors or Ork Burnas, they could burn large swatches of enemies or focus their streams in order to cut through armor. Apart of this fancy weapon, every Pyroclast wore a suit of armor called a Mantle of Ash. Power Armour that was even more heat proof and boosted further the Salamander's heat resistant capabilities.

Ashen Circle

A specialist division that stood alongside the Word Bearers Destroyer units. Rather than being tasked with the absolute destruction of the enemy, their duty was to obliterate their enemy's culture, knowledge, and faith; everything which would give the foe the heart to continue fighting. Interestingly, this formation predated Lorgar's induction to the Legion, echoing from a time when the Word Bearers were originally the Imperial Heralds, who would bring the truth of the Great Crusade to newly encountered human civilizations.

This meant that they would specifically seek out and burn libraries, schools, churches, and government buildings as well as targeting enemy priests, officials and standard bearers so that it would be easier to replace them with the Imperial Creed when the time came for compliance. To this end they were armed with Hand Flamers and specialist Axe-Rakes, which were designed for tearing down icons and symbols, but were also good for dragging down fleeing targets. They were also uniformly equipped with Jump Packs and combined their drop assaults and charges with bursts of flame.