Daughter of the Night by Gemma Vale

Seren has survived nineteen years in the Hollow by keeping her shadows hidden.

But when they take a life she never meant to lose, she’s dragged into the gilded city above — a prisoner of the very institution hunting her kind to extinction.
Kael is a scholar of the shadows, sent to observe and contain. What he finds instead is a girl whose darkness doesn’t just answer her — it remembers something older than either of them.
As ancient shrines awaken beneath the city and a goddess stirs in the dark, Seren begins to understand that her shadows were never a curse. They were always a calling.
And the light will do anything to silence it.
She was never meant to survive. She was meant to become something else.