The Dark Caste (also known as the Bandit Caste) refers to the lowest caste in Clan society, that of the outcasts and bandits. This caste is not recognized by Clan society, yet it exists. It supports the failures of Clan society, the unwanted, and the rejects. The dark caste has no voice in Clan affairs because their group is not sanctioned.
Clan literature is filled with stories of malcontents who fled to the bandit caste and came to unsavory ends. Bandit caste atrocities are the stuff of legends, but it is difficult to believe that the caste is strong enough to be more than a minor nuisance to the Clans.
It is unknown whether the Clans allow the Bandit Caste to exist or not; some cynically believe that it is fostered on purpose to provide both an outlet for those disaffected by Clan society and as a built-in enemy for training purposes. Regardless, the caste does serve two purposes. First, it is incentive to children to excel in their caste's profession to avoid needing to join the Bandit Caste if they are not deemed useful to the Clan. Lastly, it provides an enemy, although a somewhat weak enemy, for the Warrior Caste to sharpen its skills against when a Bandit Caste settlement becomes strong enough to become enough of a nuisance.
Warriors who fail to test up or who grow too old to be of service sometimes slip away to this caste, as do scientists whose discoveries are deemed counterproductive to the Clans.
 Post Jihad
In the wake of the invasion of the Inner Sphere, the Clans created occupation zones and attempted to scour the local pirate populations away. As the Clans became more settled in their occupation zones, the Dark Caste began to establish a presence within those zones itself; initially, the Dark Caste clashed with those pirate elements that had survived, but it wasn't long before the two groups began to work together.
The Jihad gave the opportunity for both groups to strike at their common foe, claiming many small victories and more firmly establishing their presence; by the beginning of the 3080s, the Clan-occupied zones were home to more pirate-related activity than any other area in the Inner Sphere. Those pirates who survived the initial Clan attempts to destroy them have evolved to face the threat posed, and the Clans have been unable to eradicate the Dark Caste despite centuries of effort, making it likely that the two groups will continue to enjoy successes working together.