Back when the Black Legion still went by the Sons of Horus, the Justaerin were a formation of black-armored Terminators that represented the pride of their legion, tasked with being the 'tip of the spear' of assaults where the fighting was hardest and were the favored of Horus' legionaries. Making up part of the prestigious 1st Company (the other half being the Catulan Reavers), they were usually went about destroying the heart of an opposing army or conducting a decapitation strike of the enemy's command structures. Heck, their leader (a certain despoiler) would eventually turn out to champion his father's cause in bringing down the Imperium.
Following said revolution, the death of their Primarch and the reformation of the legion, the Justaerin had been effectively replaced by the Bringers of Despair. The Justaerin, or at least their remnants, still serve as part of the Black Legion's Terminator elite.