Stalk the Dark by Debbie Cassidy

A Vampire King without a heart needs a watcher who guards hers dearly, but I didn’t expect that watcher to be me.
The Order of Helsing have protected the innocent from the Vampire King’s ravenous hunger and cruel games for centuries.
He can’t be killed.
He can’t be stopped.
Because this ancient being has no master.
Once every century he rises from his vampiric sleep to feed, and the Order assigns an operative to watch over him.
This century, I’m that watcher.
I can’t say I’m thrilled. I have a disgust for suckers, so babysitting vampire royalty is not on my to do list. But when the Order says jump, you don’t ask why.
Ezekiel’s cruel beauty makes him the perfect predator, able to reel in his prey with pretty words and promises. Tempering his hunger until he falls back into a vampire coma won’t be easy. Others have tried and failed.
I’m determined to avoid his kill list, and with the help of the two rogue vampire hunters assigned to assist me, I might just succeed.
But the more time I spend with Ezekiel the more I’m drawn to the darkness inside him, because there’s a darkness inside me too. One which I’ve hidden for as long as I can remember. But now that it wants to come out and play, I’m afraid it isn’t just his hunger I’ll need to temper, but also my own.