Vanguard Deployment: This unit can deploy outside the deployment zone.
Hide (forest): This unit can hide in forests until enemy units get too close.
Dark Riders (Repeater Crossbow) is a Dark Elves cavalry unit introduced in Total War: Warhammer II. Being everywhere and nowhere is their signature skill, with volleys of fire to harass the foe.
In the grim days when armies of Daemons besieged Ulthuan, keen-eyed messengers from Nagarythe kept guard for any signs of a daemonic intrusion. These Dark Riders wore cloaks of black feathers, invoking the raven-headed god Nethu to keep them hidden from the eyes of the enemy. During the civil war, the Dark Riders earned a more sinister reputation. They travelled ahead of Nagarythe's hosts, sowing terror and confusion wherever they rode, burning villages and driving their people into the wilds. In battle, Dark Riders take delight in skirting the enemy flanks to attack war machines and cut lines of supply. They revel in the prospect of running down terror-stricken victims, dragging out every moment of wild panic as long as possible before delivering the final heart-seeking strike.
↑Armour-Piercing Missiles: The damage of armour-piercing weapons mostly ignores the armour of the target, making them the ideal choice against heavily-armoured enemies. They are often heavier and attack at a slower rate though, making them less efficient against poorly-armoured targets.
↑Vanguard Deployment: This unit can deploy in an expanded deployment area, allowing it to start the battle within striking distance of the enemy - or somewhere unexpected.
↑Very Fast: This unit can run circles around most other units, taunting and harrassing the enemy or evading its missile fire.
Dark riders (repeater crossbow) are a niche unit to have at all stages of a campaign. Suffering from low range, low rate of fire as well as low model count (for a ranged unit), they are best used for harassing or taking out artillery pieces. Their low melee attack and defense and sub par charge bonus make them a mediocre choice for engaging enemy missile infantry in melee and absolutely horrendous at it in higher battle difficulties. They will lose all sustained ranged duels due to lack of shields. Regardless, they are still a good unit for flanking and shooting on the back of melee units engaged by your own melee troops, that is if you have no enemy units focus firing you!
Despite the downsides outlined above, they are unique among missile cavalry for firing 2 projectiles per unit per volley. They also have significantly more armour-piercing than any other similar units (only Mogrubb's Mangy Marauders) comes close. This means they can output a significant amount of armour-piercing damage in a single volley before needing to kite back and avoid reprisals from the enemy and wait out their lengthy reload times.