Sometimes seen as a curse and sometimes as a boon, harnessed for incredible power the story of the Black Children is a young one to the world of Isekai and is considered to be a direct product of the Eldritch War.
During the Eldritch War, titanic creatures of otherworldly origin wreaked havoc in Isekai. They fed on and destroyed the world itself, on the living beings, the terrain and the natural magical energy within the world. The war that ensued taxed all of these things and left an undeniable mark, lands that would never support life and the magical energy of the world seemed to have a scar that manifested in the stories called the Black Children. These are creatures, whether of the Divine, Elder or intelligent races of the world or not they are distinct and unique among life forms in the world. They have magical traits often unheard among the races, some of them being things that have not been learned by scholars and some of them are completely unique with traits that will never be exhibited by other mortal beings. Those that would harness these powers for terrific or terrible deeds view the powers as a blessing but a majority of the population views them as mutants, freaks of nature or powerful mystic beings in the most favorable circumstances.
The Black Children have only been born since the end of the Eldritch War, a few appearing within the 100-year span after the war but being relatively undocumented due to a lack of association with the previous war. In current years the number of the Black Children hasn't become more frequent but rather nations more actively search for them and categorize them. It is said that no more than 1 every 100,000 of a race could be affected by this scarring of the magical energy of the world, sometimes less. In many cases, these people have become powerful adventurers or risen to power within their respective countries but their reception still varies and the world knows little about them beyond myth and story.
The Black Children are varied in origin and ability, there seem to be only a few rules governing their existence which is what has made their initial emergence into the world more easily understood as a folk tale or legend. The first rule that is believed to govern their existence is that they will never be born in large cities or heavily populated areas, for some reason they emerge into the world more humbly that some would expect. The second rule is that they have no aptitude for magic, despite their attempts and pleas to the various gods of Isekai they will never have the ability to wield magic nor touch the natural power of the world through the use of spells. While it isn't a rule there is also a trait that all of the Black Children share, when viewed through the use of a detect magic spell or through the eyes of a creature that naturally sees magic these people will appear to have a black smoke lingering about them. Creatures that have a strong affinity with magic like dragons that can smell magic appear to dislike the "smell" that these people have, most magical creatures attempt to avoid them.
The first appearance was a year after the end of the war, a child was born among the race of Elves. A boy that was exceptionally bright and talented, that all assumed would become a master enchanter or a wizard. When he came of age he showed no skill for magic, no ability to wield the pen of the Elves and his talent for physical combat was lacking. This plagued the boy and strengthened his desire to learn, he delved into a science of the natural world that would provide a clear path for him: Alchemy. In this, he found success, potions, understanding of compositions and the understanding of science in tandem to the existence of magic in the world. His studies revolved closely around the use of metals and herbs in combination to emulate the effects of magic without the use of it and he contributed to groundbreaking work in the field of temporary augmentation through the use of potions. However, his existence as the first of the Black Children comes from his culminating work that is called the Circle of Reaffirmation. It is rumored that this circle proved that science was equivalent to magic and able to negate all forms of magic, enchantment and even Elven script within the circle without the use of magic to overpower and nullify them. The technique to create this circle was kept secret from all, even the Elven King, and the Alchemist died soon after.
The Black Children have been appearing without any specific concentration in region, time period, race or lineage starting with the first year after the Eldritch War ended.