The Battle Born is an infantry unit that is created as a result of the Proliferator's "Parasitism" capacity. Battle Born are added to an army at the end of a battle if the Proliferator successfully affected an enemy unit with Parasitism, and that unit died in battle, or can be added to a city's garrison by using a Recycled Stockpile on that city during a Dust Eclipse. Battle Born always spawn at Level 1 equipped with the highest-tier Iron or Dust Claws, Helm, Cuirass, and Greaves available, and cannot be retrofit.
One of the unique properties of the Battle Born is that they ignore Army and Garrison size restrictions; even if your max army size is only 4, you can still have dozens of Battle Born units in the army, so long as Proliferators continue to kill units with Parasitism - Battle Born units must be "spawned" into armies to ignore the max, and cannot merge into armies that are over the maximum. However, having more units in an army than you are allowed comes with an exponentially increasing penalty added to the army's upkeep, with the penalty for having 2 units above max being 40 per turn, but having 12 units over max is about 500 per turn. As such, this property of the Battle Born should be used with restraint, or you should heavily invest in reducing the cost of Military upkeep, either by the Empire Plan, Hero Skills, the Reliquary of Auriga, or the Cannon Fodder Necrophage Technology.
Created from their fallen enemies on the field of battle, these soldiers of the hive do not feel pain, pity, or remorse.