At the very end of the first season, viewers see General Kirigan walk out of the Shadow Fold with shadow-like creatures forming and following behind him. Many have assumed that they are the volcra since they came out of the Fold, but they are entirely different creatures that are just as monstrous.

In the novels, those monsters are the nichevo'ya, an entirely non-human creation made out of the Darkling's own power. They are unnatural beings, and creating them was not supposed to be possible with Grisha powers, but by the beginning of Ruin and Rising, readers learn that by giving Alina the collar, the Darkling became powerful enough to create a "life" of sorts with the nichevo'ya.

This is not explained in Season 1 of Shadow and Bone, though it's presumed that the monsters' origins will be expanded upon more in Season 2. While both the volcra and the nichevo'ya are created out of darkness, only the volcra had human origins.