The Second Death of Locke Book by V. L. Bovalino

The Second Death of Locke follows Grey Flynn, a "well" who supplies the power to her mage, Kier. They share a deep bond built over a lifetime of friendship and codependence. When they are sent on a mission to protect the alleged heir of the lost kingdom of Locke, Grey and Kier find themselves embroiled in a war and must confront long-hidden secrets.