Miners of the Tempest by Penny Moss

In the depths of the Tesera Mine, Oliver’s hope of finding Tau again fades with each passing day. Just as despair consumes him, his Sentinel returns, only a shadow of the mighty being he once was.
Oliver swears revenge against those responsible, but there’s no time to plan or savour being reunited, as a deadly mistake forces him and his friends to escape.
With no other choice, and a weakened Tau, Oliver traces the steps of his missing teammate to war-torn lands. He’s wholly unprepared to discover the toll exacted by mining for magic, and it’s worse than Oliver could have ever imagined. His hunger for revenge becomes a desperate need to put a stop to the atrocities committed, even if it means forming an alliance with an adversary.
In a sea teeming with horrors and growing distrust among his friends, Oliver and Tau find solace only in each other’s embrace.