Last Breath by Katherine Diane

It’s been two months since the body of the Dark Prince vanished in the clawed grip of a demon lord. With no further sign of trouble, the Hush is back to business as usual. And Syren, heir to the vampire throne, is up to her old tricks—or so it seems.

When Kyr and Ronan track her down at a nightclub, Ronan isn’t surprised to see the spoiled princess backsliding into her party-girl ways, but he is surprised (and not happy) to get tasked with babysitting her. Syren isn’t happy either—because she has a reason to be spending her nights in the clubs … and it’s not to party.
But Syren isn’t the only one with secrets. Ronan is desperate to keep a lock on his. Because the weird crap that starts happening to his mind as his treatments begin to fail? Even a tough-as-hell male like Ronan can’t quite face that. At least … not alone.
As he and Syren start to uncover each other’s truths, their discoveries form a bond neither of them expected. But will it be enough to bind them together when Ronan’s most dangerous secrets threaten to tear everything apart?

As the battle between demons and vampires reaches its peak, Ronan’s past gets dragged into the present, new realities emerge—and nothing will ever be the same again.