Vaults of Obsidian: A Blackstone Fortress Anthology - Various Authors

Chasing shadows and saving the world at the age of sixty-nine? Not on her bingo card.
Former nun Sister Monica Barrett is the reluctant keeper of a stone that holds immense dark magick. She'd abandon it if she could, but it's become attached to her—literally—and there are five more like it in the hands of powerful, corrupt Mages determined to control the very fabric of the world. Even if they destroy that world in the process.
Fortunately, Monica's foes haven't been able to find the one thing they need to help them tap into the stones' powers—a mysterious being named Methuselah, who brought the fragments to Earth millennia before.
Unfortunately, neither has Monica.
On her side: a fractured secret sisterhood, an off-the-clock cop, and a tech-less tech whiz. On the enemy's? Unlimited financial resources, their own dark magick, and a host of terrifying monsters.
The race to find Methuselah is on—and the entire planet hangs in the balance.