During the Dawn War, some of the Gods took desperate measures to gain more power and warriors to aid them against the Primordials. One of The Three Who Are Hidden, Balphomar the Ironhanded, found a race of indigo-skinned humanoids dwelling amongst the reeking, acidic fens of a patch of Velnar. He captured them, brought them back to his iron fortress in the Astral Sea, and transformed them.
At the next battle of the War, the bladelings entered under Balphomar’s control. Their indigo flesh was now permeated with spikes and blades, and they fought with a tenacious ferocity that gave pause even to the Dragonborn and the Urku. Although the bladelings were clearly in torment, this seemed to only goad them on to more ferocious acts of warfare. Gods and Primordials alike felt a twinge of horror at what the War was costing them, and, indeed, this act was partly responsible for the other Gods shunning Balphomar.
Although most bladelings still reside in Balphomar’s Astral domain, some have escaped over the years. Knowing little more than warfare, they have taken to hiring themselves out as mercenaries, studying the various arts of the sword, and wandering, unable to find any home that truly feels like their own any more. Those of an arcane bent sometimes study Swordmagic, but it’s rare to find true wizards amongst them. Most who have escaped Balphomar reject the Gods in general, but some find solace in the worship of various deities. Most simply find that war is what they’re good at, so they become warriors, or, if there is no war to be found, bandits or adventurers.