1st Cairngorn Penal Legion

Origins
The mining world Cairngorn is a barren, hostile world on the outskirts of Imperial territory, located on the northern fringe of the Gothic Sector between the Segmenta Obscurus and Ultima. It's rocky, mountainous terrain features vertical, circular shafts all over it's surface, sometimes several kilometers in width and depth. It is speculated that they are leftover mining efforts of a long-forgotten alien species, though no remains of them have been found.

In mid-M38, the planet was re-colonized as a penal mining colony, the resources intended to be used in the ever increasing war-efforts of the Imperium. Thus, millions of convicts from all over the Segmentum were shipped to Cairngorn, overseen by a comparatively small force of PDF lead by an officer of the Adeptus Arbites.

The number of prisoners was severely thinned out in the first years, given the harsh conditions, accidents, cave-ins and the clash between the more and the less violent convicts. What remained were a hardy, tenacious core of men and women, slowly adapting to their environment.

Peril In The Darkness Edit
When the Imperium was thrown into disarray by the Despoiler's most recent Black Crusade, Cairngorn received less and less supplies, which brought the morale of the population to a dangerous low. Rebellion seemed unavoidable, but the prisoner's ambitions were cut short when a raiding party of Dark Eldar emerged over the planet. Slaughtering the inexperienced and under-equipped PDF, they made their way into the caves and abducted thousands of prisoners before they could organize themselves properly.

Eventually retreating deeper into the tunnels, the prisoners picked up the weapons of their former wardens and prepared what seemed to be a last stand. However, after a costly first weeks, the Xenos seemed to suddenly fall into chaos after the prisoners managed to crush a particular large force of them in an artifically created cave-in. Unknown to them, they had eliminated one of the Raiding Party's leaders, which lead to vicious infighting over the succession of his possession.

Attempting to emulate the tactics of their invaders, a series of desperate counter-attacks was launched and the prisoners actually managed to drive the Dark Eldar back, although suffering great losses. This event, paired with the eventual arrival of an Imperial force sealed the fate of the Xenos and they were largely annihilated when they attempted to retreat into the Webway.

The relief of the convicts did not last for long and neither did their celebration for their presumed liberators. The imperial force had not come to their aid as it first seemed. In fact, they had been entirely unaware of the state of Cairngorn and the presence of the Dark Eldar. They had come to conscribe the prisoners into a regiment to aid in the effort of the Indomnitus Crusade.

The regiment was raised as a penal legion with a focus on supporting roles and sabotage, making use of the prisoners' expertise with explosives and combat in tight surroundings.