Diadem of Blood and Bones by Ripley Proserpina

Sacrifice….
Transformation…
It all comes down to the final battle…
Forced to choose between her life and the lives of the vampires she loves, Briar made the ultimate sacrifice. For her, a world without Marcus, Hudson, Sylvain, and Valen was inconceivable.
But her vampires didn’t accept her decision, and at the last moment, transformed her from woman to immortal.
Awakened to a new, utterly different life, the sun can finally shine on Briar Hale. With eternity in front of her, all she sees is love and hope.
So when the creatures of darkness threaten everything she holds dear, Briar will not surrender. She’s stronger now and she has a vampire inside her ready to go to war.
Can Briar hold onto herself while protecting those she loves? Or will she give into her new power, and become her worst nightmare?