Winter Fire (#3, Witchling Series). The Witchlings Series, #3

A beautiful fire witchling with a dangerous secret .A race to save the Light from permanent Darkness .A Dark threat unites with an ancient evil .If the Master of Light, Beck, has learned anything over the past few months, it's never to trust a pretty girl - or his own judgment, the source of several messes he's working hard to right. He saved his brother and girlfriend from Darkness but faults himself for not protecting them in the first place, like he's sworn to.
The Light is receding again, and his earth magick warns him that an ancient danger has re-emerged in the hands of a beautiful, conflicted girl who's caught between Light and Dark. Drawn to her beauty and gentle heart, Beck is astonished to learn her magick will do more than set his blood on fire. Like his twin, the Master of Dark, Beck has a counterbalance, a woman who can help him protect the Light - if she doesn't fall to the Dark like the fire witchlings before her.
Hurt too often by bullies in her own life, Morgan tries to protect those around her from similar pain. She and her brother arrive at the boarding school in the Rocky Mountains, only for Morgan to make enemies instantly with Dawn, the Dark witchling who tried to kill Summer twice. But Morgan is hiding something, a source of ancient evil, known as a soul stone, passed down through a thousand years of fire witchlings.
Fire repels the coldness of the Dark, and only fire witchlings can safeguard the stone. Taking it into the middle of the source of Light awakens its powers. It finds an ally in Dawn and teaches the Dark witchling how to evade and seek her revenge upon the Masters of Light and Dark. Morgan grabs the soul stone and runs, only to entangle Summer and Biji in her desperate attempt to protect Beck. Hidden from the Masters of Light and Dark, they must find a way to escape Dawn with their lives - and souls.