Superior Saturday

On the sixth day, there was sorcery.
Arthur Penhaligan has wrested five of the Keys from their immortal guardians, the Trustees of the Will. But gaining the Sixth Key poses a greater challenge than any he has faced before. Superior Saturday is not just one of the Trustees – she is also the oldest Denizen and the most powerful and knowledgeable sorcerer within the House. She has tens of thousands of sorcerers at her command, and she has been preparing her forces all along for the Will's escape and the activities of the Rightful Heir.
As Saturday's schemes become evident, Arthur is beset on all sides. The House is being destroyed, and only the power of the Keys can hold back the tide of destruction. Arthur's home city is under attack. His allies are unreliable. He can't even get into the apparently impregnable Upper House . . . and even if he does, finding the Sixth Part of the Will and gaining the Sixth Key might not be enough to counter Saturday's bid for ultimate power.
Can Arthur survive?