"It was to store power," Lydia explained. "The Order of the Rose was...a kind of coven, I suppose, although that feels like the wrong word." More like a perversion of a coven. "They had this great working, like a magical tapestry I said, that all their members were a part of, and they invited me to join before I quite knew what I was doing...but I had to give blood to join, and I was sick of blood by then." Ominous. "So I gave the illusion of a drop of blood instead. And then later, I found myself quite at odds with them." She considered how to explain it, then said, "In order to control who could use the power they had saved up, they had to be able to control the witches. But I hadn't committed to it in blood, so I was in it, but not. So they couldn't control me. And I looked at it with the mind's eye, and I yanked just so, and the whole thing unraveled."

That was actually fairly concise, all things considered. Well done, Lydia!