The Saint's Magic Power is Omnipotent (Light Novel) Vol. 4

DARK HEART OF THE FOREST
At long last, Sei knows how to control the power of the Saint that resides within her...which means it's time to wield it. She must cleanse the forests of Klausner's Domain, or her new friends will never again know peace and prosperity. But Sei's first foray into the forest depths nearly ends in disaster. While she and the knights await reinforcements from the capital, Sei has to wonder: what if the Saint's magic isn't enough? What must she be ready to sacrifice to save the people of Klausner?