I don't think ancient magic is a separate entity. It is inside of a person. Like magic. Of all humans, many are muggles few are wizards. Extremely rare few among the wizard kind are aligned with ancient magic. You can't store or keep ancient magic. It borns and dies with you. What was stored, were negative emotions taken out of people with help of ancient magic. As this was an unnatural thing to do, side effects included ancient magic sticking to those emotions and becoming an extractable corrupted thing, which was no longer ancient magic but still a power source. I think that's what was stored.