Blood Servants are mutated dogs engineered by Twilight Elf mad scientists.
Fast, gruesome beasts made by the Twilight Elves. Created only to seek and destroy.
Blood Servants are fast, come in large groups but are fairly weak individually, only having 200 HP and so-so damage. Their danger mostly comes from their sheer number, which allows them to slip through a defence without enough AoE damage; even then, their high health (for a fast enemy) makes them especially dangerous in large groups.
Compared to Blood Gnolls, Blood Servants are very similar in terms of speed and attack, although they have much lower HP but no increased vulnerability to mages.
These enemies, despite the fearsome description, are not especially strong stats-wise. Unlike the Worgs and War Hounds, they possess no magic resistance, and are unarmored as well, so they are fully vulnerable to your towers. The damage they deal is low, too, combine with the improved armor of Elven units, soldiers of Wardens' tier or higher can hold them back significantly long. Then, one or two good advanced towers can obliterate them easily. Druids will destroy groups, trading fire rate for solid AoE damage, while Mages and Archers will weaken them rapidly a few at a time. Generally, Mages and Archers are more reliable, but you would want at least one spot for an advanced Druid in case of mad hordes of Blood Servants.