Evergreen: Mer Tales #2 - David Annandale

She may be too old to start a fight, but she's not too old to finish one.
With sixty-nine years and two black belts to her name, former nun Monica Barrett has never backed down from a fight. So when a woman turns up on her doorstep with an attacker on her heels, Monica is quick to step in.
But this assailant has powers—he moves with uncanny speed, strikes with brutal accuracy, and isn't after the woman at all. Instead, he wants what she has: a smooth, flat piece of rock that she throws to Monica.
The instant she touches the stone, everything Monica has ever known turns upside down. Inhuman power surges through her, and she destroys the attacker. Horrified, she is determined to rid herself of the stone and whatever it holds.
Then more attacks follow, and suddenly, the stone's dark power is all that stands between her and losing everything she holds dear. Like it or not, Monica has become its keeper .
And its wielder.