Justaerin
Back when the Black Legion still went by the Sons of Horus, the Justaerin were a formation of black-armored Cataphractii 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.
They should not be confused with the Luperci, although Horus didn't really make things better when he sometimes used both the Justaerin and Luperci as Honour Guard-like roles.
Overview[edit]
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. The name Justerin is itself derived from a powerful gang on Cthonia in the days before the Imperium. So yeah, literal gangsters.
Following said revolution, the Justaerin were effectively wiped out; fifty of them were killed in a fight with some Dark Angels on the far side of the galaxy, and nearly all of the rest died in a failed sneak attack on the Saturnine Wall during the Siege of Terra. Along with the death of their Primarch and the reformation of the Sons of Horus into the Black Legion, this saw the Justaerin effectively replaced by the Bringers of Despair, formed from thirty Justaerin survivors who became possessed jointly (it's complicated, see their article for information). The Justaerin, or at least any remnants who weren't possessed and then after that their replacement/supplementary promotions over the years, still serve as part of the Black Legion's Terminator elite, though as simply the premier Terminator formation instead of the Honor Guard.
As the elites, the Justaerin was well armed and armoured. As aforementioned, they wore Cataphractii Pattern Terminator Armour, and were equipped with a Banestrike Combi-Bolter, Multi-Meltas, Plasma Blasters, twin-linked Autocannons, Volkite Chargers and the rare Combi-Volkite. Melee wise, they can choose from the Carsoran Power Axe, to a pair of Lightning Claws, or a Power Maul and a Power Lance.
Crunch[edit]
On the Horus Heresy tabletop, they were an elite set of Cataphractii terminators with WS and BS 5, 3 attacks, Chosen Warriors and Furious Charge (1). They can be selected as a Retinue squad for a Master of the Legion, not taking up a Force Organisation Slot. Interestingly, they have an additional line to their Retinue rule where if their Leader can Deep Strike, then they are allowed to Deep Strike too, which is especially useful for characters like Horus and Abaddon.
They come with a Banestrike combi-bolter and Carsoran power axe as default but can trade up for the usual goodies, including a Multi-Melta. Do keep in mind however that they are used as an expense benchmark for a reason. At 50 points a model at base, they can very quickly become a huge points sink. Unlike many expensive units in the game however, they are well worth the cost so long as you use them wisely. With the proper kit (a couple of Melta guns, a few Thunder Hammers, Banestrike bolters ect), they are a threat to practically every type of unit in the game.