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 forbidden and proscribed technologies, including horrendously radioactive and toxic weaponry originating from Terra's accursed Old Night epoch, along with such nasty gear like the now-lost Phosphex weapons (bombs in this case). This horrific stuff was as devastatingly effective as it used to be in Terra's dark past, but was so morally questionable and considered dishonorable in general from the P.O.V. of even the most pragmatic Legions (and that's a lot), that they fell into general disuse--after all, what's the point in using weapons like these to reduce enemies to radioactive and/or toxic ash when the ground you gain becomes so inimical to human life as to be useless to you or most anyone else for centuries or even millenia, as had been the case during Terra's Old Night?
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 against Xenos (who had no chance of joining the Imperium at large) or dug-in humans who would not submit and held nothing of strategic value but still needed to be made an example of, though most of the Legions either shunned them, as mentioned previously, or simply didn't have them at all. Fittingly, the only real command personnel who would regularly deploy alongside units of generic Destroyers would be Moritats.
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 they required cybernetic replacements. The fact that being assigned to a Legion's Destroyer Corps virtually guaranteed a particularly nasty and slow death, even without any efforts on the part of the Imperium's enemies, meant that such an assignment was a de facto death sentence for many legionnaires who either stepped out of line or got on the wrong side of their Primarch (though very few tougher-than-nails bastards, such as Crysos Morturg, would survive for a long time and even thrive in such a lethal occupation).
Needless to say, the one Legion that would be associated the most with these guys would be the Death Guard. Having their superhuman resistances further boosted with capabilities of Mortarion's geneseed, the Death Guard would become the experts on biological warfare and sent 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.
A brief list of Legiones Astartes that had special formations or specialized variants of destroyers in their forces follows below.
"We are here! We are death!"
- – The battle cry of the Dreadwing
The Dark Angels had their own formation built around Destroyers called the Dreadwing, which is not surprising since the Emperor had the First Legion originally outfitted with stores from Old Terra, including the formerly-forbidden ones, in his crusade to first take back the Sol System and then conquer the Milky Way Galaxy at large. Due to Destroyers being part of their "standard operating procedure" right from the beginning (since so many of Sol's planets were already naturally lifeless to begin with, ergo having no natural biospheres to taint with the use of Destroyer weaponry aside from Terra's), the Dark Angels were no slouches in deploying Destroyers when the situation called for it, and even armed them with some of the most devastating vehicles and ammunition the proscribed vaults of Old Terra had to offer, such as Phosphex and Vortex weapons. This extended even to heavily armoured vehicles, such as Fellglaives equipped with Warp Cannons, Vindicators firing Vortex Shells, Whirlwind tanks firing radiation missiles and at least one vessel with an Annihilator Cannon. As such, the Dreadwing got the call whenever the Legion needed some really nasty shit to get rid of a target immediately and collateral damage was not an issue. However, a contentious deployment of this formation got the Primarch Lion El'Jonson and his Legion kicked out of Imperium Secundus. After the Horus Heresy, it is believed that the Dreadwing were formed into the Star Phantoms Chapter.
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.
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.
In Lorgar: Bearer of the Word, it's revealed that the Ashen Circle were all hardcore Imperial loyalists who, for obvious reasons given the whole faith-obliterating thing, were extremely unhappy with the religious nature of the Word Bearers. When the smackdown came at Monarchia, they were pretty pleased that the Word Bearers were going to get back on the track of obliterating faith as opposed to engaging in it, but their excitement at the turn around didn't last long. Kor Phaeron and Erebus promised them the chance to burn a bunch of Lorgar's religious works, which caused much rejoicing among the Ashen Circle. However, joy soon turned to ashes (see what we did there?) in their mouths when the obliteration opportunity turned out to be a giant trap and they were slain to a man before they even really realized what was happening. Then Kor Phaeron and Erebus had their remains torched to hide the evidence, which is a classic bit of hamfisted irony from Black Library.
The 22nd Chapter of the Ultramarines Legion, known as the Nemesis Chapter, was a conglomerate chapter consisting of a number of specialized units, most famously two whole companies of Destroyers. Guilliman didn't care for them very much, given their specialization in completely wiping out anything that ever lived on a planet went against his general desire to colonize and civilize worlds as opposed to just nuking them back to the Stone Age. In order to try to get them back into the fold, Guilliman appointed a new, young hotshot Captain named Eleon Iasus to try and get them moving towards the Ultramarines ethos. It sort of worked, despite the fact that some of the captains in the 22nd went so far as to outright disobey Iasus during the Battle of Thoas (resulting in a geological catastrophe that killed the disobedient officers, whoopsy daisy) and Guilliman reluctantly began to incorporate them a little bit more into the battle lines of the Ultramarines as the Heresy got worse and worse.
Some elements were present at Calth, were they mustered in the highlands far away from civilian population centers. At Pyrrhan, the 22nd led the way laying waste to unending ranks of daemons with their devastating weaponry, because sometimes the best way to deal with Chaos is just to coat it in Phosphex and radiation until it stops moving. They also participated in boarding actions against the Word Bearers at the Battle of Anuari. After the splitting of the Legions, the 22nd may have gone on to form the Nemesis Chapter, a Second-Founding Ultramarines successor chapter. Maybe they kept their love of horrifically devastating weaponry, but we don't know. Their last recorded action was when they committed their entire chapter to fight the Necrons in the Orphean War because sometimes the best way to deal with Necrons is just to coat them in Phosphex and radiation until they stop moving (or phase out or pray to Ra or whatever they do these days).