In the beginning of the series, it is assumed that the Yoma have always existed and the Claymores were created to fight them but it's never mentioned where they come from. Later on, it's revealed the Yoma were created by the Organization to fight a species of dragon descendants.
How is it that the Organization creates these Yoma?
From chapter 126, it's revealed that the Organization
creates a parasite from combining flesh from a Dragon's Kin with flesh from a captured Dragon. This parasite will infect the brains of humans and turns them into Yoma.
Miria had previously speculated that the there was a war raging outside of the land where the Claymores were and that
the two sides were the people who run the Organization and the descendants of Dragons and suspects that Yoma were created by the Organization. In chapter 126, she learns from Rimuto, an Organization member, that the Yoma were created by the Organization as a cover for them to develop Claymore. The Organization's goal was to create a weapon to fight the Dragons and Dragon Kin that could awaken but then revert back to human form. Previously, awakened warriors couldn't revert and they just left them in battle to die.
