Heiress of Secrets - Heiress of the Seven Cities, #3

After a monster gifts her with magic always denied her, a spurned queen becomes a mistress of secrets... Once, I loved a silver-haired mute boy who'd been found by the sea. Now, wed to the highest ruler of the known world, I am a spurned and barren queen, unable to bear the septuplets that should save our world. And yet my king refuses to let me escape, despite my repeated attempts to have our own daughter slain.
At last, I seek to flee by taking my own life. But inside the glass shard I hold to my wrist, a gray, fanged monster appears. Our ancient enemy--the shadow king--offers me a chance: if I vow to him some unspecified future favor, he swears that I will bear the septuplets. In return for my willingness to bargain, he fills my palms with magic: he gifts me a vial that swarms within with golden strands which enable me to spy on anyone I wish.
I accept. But after I return to the castle to spy on those I hate, I discover uncanny plots unfolding in every corner of castle and city. Not only that, but I find myself verbally sparring with a trickster mage who appears in my private chambers whenever he likes, I must endure my faithless magnate weaseling his way back into my life, and my once love, my only love, long dead, resurrects in the most terrible, unexpected way.
And all the while, the shadow king's bargain awaits, always, ever at my heels.