"Ruthless" by Candice Pedraza Yamnitz

The novella Ruthless is a prequel to Unbetrothed, which I reviewed here a couple years ago.  It focuses on Cottia, the mother of Unbetrothed's heroine, and her late teen years when she was an assassin.  I kid you not!  She is held in a sort of thrall by an evil man who makes Cottia use her magical talents to kill people on his orders.
Cottia is desperate to leave this lifestyle.  She believes her master when he says that if she can kill a certain person at a big party and use is death to start a ...