An Oath of Brothers (The Sorcerer's Ring #14) by Morgan Rice

In AN OATH OF BROTHERS, Thorgrin and his brothers emerge from the land of the dead, more driven than ever to find Guwayne, and set sail across a hostile sea, leading them to places beyond their wildest dreams. As they come ever-closer to finding Guwayne, they also encounter obstacles unlike ever before, obstacles which will test them to their very limits, which will call on all their training and force them to stand together as one, as brothers.
Darius stands up to the Empire, boldly amassing an army as he frees one slave village after the next. Facing off against fortified cities, against an army a thousand times his size, he summons all of his instincts and courage, determined to survive, determined to win, to strive for freedom at any cost, even at the cost of his life.

Will Thorgrin find Guwayne? Will Gwendolyn and her people survive? Will Godfrey escape? Will Erec and Alistair reach the Empire? Will Volusia become the next Empress? Will Darius lead his people to victory?