shield crystal

"... really no point to doing anything but waiting," Ebenezar said. "Sure, they're under a roof, but it's leaking. And if nothing else, morning should take care of it."
"Ai ya," Ancient Mai muttered. "I'm sure we could counter it easily enough."
"Not without risk," Ebenezar said in a reasonable tone. "Morgan isn't going anywhere. What's the harm in waiting for the shield to fall?"
"I do not care for this place," Ancient Mai replied. "Its feng shui is unpleasant. And if the child was no warlock, she would have lowered the shield by now."
"No!" came Molly's voice. It sounded weirdly modulated, as if being filtered through fifty feet of a corrugated pipe and a kazoo. "I'm not dropping the shield until Harry says it's okay." After a brief pause she added, "Uh, besides. I'm not sure how."
A voice belonging to one of the Wardens said, "Maybe we could tunnel beneath it."
I exhaled slowly, licked my cracked lips, and said, "Don't bother. It's a sphere."
"Oh!" Molly said. "Oh, thank God! Harry!"
I sat up slowly, and before I had moved more than an inch or two, Injun Joe was supporting me. "Easy, son," he said. "Easy. You've lost some blood, and you got a knot on your head that would knock off a hat."
I felt really dizzy while he said that, but I stayed up. He passed me a canteen and I drank, slowly and carefully, one swallow at a time. Then I opened my eyes and glanced around me.
We were all in the ruined cottage. I sat on the floor near the fireplace. Ebenezar sat on the hearth in front of the fireplace, his old wooden staff leaned up against one shoulder. Ancient Mai stood on the opposite side of the cottage from me, flanked by four Wardens.
Morgan lay on the bedroll where I'd left him, unconscious or asleep, and Molly sat cross-legged on the floor beside him, holding the quartz crystal in both hands. It shimmered with a calm white light that illuminated the interior of the cottage much more thoroughly than the fire did, and a perfectly circular dome of light the size of a small camping tent enclosed both Morgan and my apprentice in a bubble of defensive energy.
"Hey," I said to Molly.
"Hey," she said back.
"I guess it worked, huh?"
Her eyes widened. "You didn't know if it would?"
"The design was sound," I said. "I'd just never had the chance to field-test it."

Turn Coat Chapter 46, Page 458-459


"Now," Ancient Mai said, turning her focus back to me. "You will instruct your apprentice to lower the shield, or I will tear it down."
"Might want to take a few steps back before you do," I said. "If anything but the proper sequence takes it apart, it explodes. It'll take out the cottage. And the tower. And the top of the hill. The kid and Morgan should be fine, though."
Molly made a choking sound.
"Hngh. Finally made that idea work, did you?" Ebenezar said.
I shrugged. "After those zombies turned up and just hammered their way through my defenses, I wanted something that would give me some options."
"How long did it take you to make?"
"Nights and weekends for three months," I sighed. "It was a real pain in the ass."
Turn Coat Chapter 46, Page 462