Hollow Huntress (Death Duet #1) by Kathryn Breaux:
 
"Careful of the Grim” is etched into the cold stone walls of the Halcyon City Prison—a warning, threat, and reminder that we all die.
 
But what about living? And which one should we fear more?
 
Mara Castten is an assassin—intelligent, calculated, and lethal. Being the top female angel within her Order demands it. She has built her reputation by being ruthless and wild, earning a nickname widely known and feared by all.
 
Mara has spent over three hundred nights imprisoned after completing an assignment she should have gotten away with. Now she is shackled—by chains that ground and keep her restrained—while they drain her magic, which desperately yearns to burst free. Over the past year, Halcyon City has continued to thrive, and the Lyre Order has operated without her. But the unsettling news of another Order gaining prominence is spreading quickly, giving the four Orders no choice but to band together for the first time in centuries.
 
They need her, and in her desperation to be free, she’s willing to do whatever’s asked of her. Or so she thinks.
 
When she returns to civilization, things aren’t quite as they appear, and when she’s forced to team up with someone she’s despised for years, things only get worse.   
 
That always seems to happen when demons get involved.   
 
Now returned to the world she once knew—once thrived in—Mara Castten faces receiving her next assignment. Having taken enough lives to keep the Grim Reaper in business, she’s now being asked to do the unthinkable—kill him.   “Careful of the Grim,” they whisper as she walks by, but… “Fear the Hollow Huntress,” she’ll make them scream.