The origins of the Destroyer Squads lay in the conduct of the Unification Wars on Terra in the late 30th Millennium, but surprisingly not with the forces of the Emperor but with the armies of His enemies. Considered dishonourable by some Space Marine Legions who refused to make use of them altogether, the Destroyers were equipped with and expert in the use of otherwise proscribed and forbidden weaponry, including certain weapons of mass destruction like chemical and tactical nuclear weapons.
Alongside certain factions of the Mechanicum, only Astartes Destroyer Squads were sanctioned to use such forbidden weapons within the forces of the Imperium by the Emperor's command. Radiation-based weapons, bio-alchemical munitions and the burning horror of Phospex were amongst the components that comprised the Destroyers' dark arsenal -- weapons which irrevocably tainted the ground on which they were used. During the long nightmare of the Age of Strife, such weapons of mass destruction rendered many once life-sustaining human colonies uninhabitable charnel houses.
Destroyers were marked by their fire-blackened and chem-scalded Power Armour. These Space Marines were often shunned and deemed somehow tainted by their fellow Battle-Brothers in many Legions. This was because Destroyers were considered only a necessary evil of the Great Crusade, although the effectiveness of their relic-weapons in wiping out particularly difficult foes could not be denied.
The drawbacks of using such deadly weaponry was the inevitable physical toll it took on a Legionary's body. Despite their genetically-enhanced Astartes physiology, Destroyer Squad Space Marines often displayed the physical affects of long-term use and proximity to such toxic weaponry, including: radiation-induced cellular degeneration, abnormally pale skin that displayed unhealthy lightning-storms of veins and blood-bruises staining the flesh, yellowed-jaundiced eyes, and aggressive and rare forms of cancer which often resulted in the replacement of the affected organs with augmetic replacements.