The Hidden Key (Second Sacred Trinity)

Be careful what you wish for. Winnie's arrival from boarding school to sunny Sydney, thrust into the sinister world of the Crone, was further from an uplifting homecoming than she could imagine. As the last Keeper of the witch-demon's stolen possession, Winnie must hide the poisonous Stone, exposed and vulnerable until she does. And her enemy is poised to break free, her swarming followers on the hunt.
Legend forecasts the final chapter nears, heralding the Stone's triumphant return and the obliteration of the feeble, ailing Trinity who've stood in their way for a thousand years. But allies exist in the most unexpected places. For the first time in millennia, the Sacred Trinity are recruiting. Satan's mistress is coming to take back what's hers and just maybe, Winnie will be ready. This is no fairy tale.
(Second Sacred Trinity, First: The Crone's Stone)