Shadow Princess by Caroline Peckham & Susanne Valenti (Zodiac Academy book 4)

The shadows are in my blood.

I can feel them crawling through me, slithering under my skin and taking me hostage.

But the darkness within me, isn't nearly as bad as the darkness that lives around me.

The four heirs will stop at nothing to destroy the lives of me and my sister. 

But they haven't figured out yet, we're the strongest creatures they've ever known...

This is the fourth book in the Supernatural Bullies and Beasts series.
Start the series now with Zodiac Academy: The Awakening