Knight of the Twin Moons by Holly Bargo

A magical union sealed with blood. A destiny plagued with inevitable misfortune.

Lord Shadow—a vicious and utterly terrifying fae assassin—is desperate for a mate. After being sent to the human realm, he finds a woman lying in a pool of her own blood. Although he knows not her fate or her purpose, he knows he must save her life, even if it comes at an irreversible cost.
And in the realm of the fae, danger is never far behind.
In the blink of an eye, her life is forever changed. She can never return to what she once knew.
After a life-altering accident, Cassandra wakes to find herself in a strange new world of magical creatures, bound by a blood union to the mysterious warrior who saved her life. Unlike anyone she’s ever met, she discovers that she is the bride to one who kills without mercy. And to make matters worse, she now possesses a mysterious power in her bones, a power that will determine the fate of the world around her.
That is, if she manages to survive.
Will she find the strength to face her destiny? Will she ever accept the fae lord who took her from everything she has ever known?