The Black Riders (Marcachi Dubh) are a small, roving fleet of Eldar Anhrathe, under charter to [REDACTED] as of this writing. Their force balance tends to skew towards more strategically mobile, less logistically-bound assets — either swift-moving jetbike-mounted raiders, speeders, or pedites. Their colors are as riotous and non-uniform as expected of Corsairs, yet a distinct preference for darker tones has been noted fleet-wide. 
 

Overall, the temperament of the Marcachi Dubh could be described as phlegmatic for Corsairs. This could be attributed to the marked presence of former members of the Eldar’s religio-military orders in their ranks, or to the philosophy they hold. Unusually for Outcasts, the several Coteries of the Black Riders encourage a sort of martial code of honor amongst their lot. While this restrains the worst impulses one might expect of their sort, it does not lessen their savagery against their foes. 
 

Hard to tell what they want, these Eldar mercenaries. Seems like they’re just here for the scrap, sometimes. They sure cut through the Partisans like Betcher’s Bugs, though![
//Anonymous]
 

Notably, the Marcachi Dubh had a marked rivalry with the Dain-Mir Eldar, for a variety of reasons. Firstly, it seems that the two had chosen opposing sides of the war, with the Black Riders choosing to seemingly throw their weight behind the loyalists. Secondly, there seemed to be the element of vendetta. At some point, the Dain-Mir appeared to have wronged the Riders, such that the wrath of the entire band was turned upon the Craftworld. Powerless to put paid to their foes, their alien psyches collectively turned to obsessive vengeance-seeking:

You pay — in blood or goods, Dhain-Muir. Since the latter is out of the question…
[[//Baron Ra’tar]]