A Grant of Arms (The Sorcerer's Ring #8) by Morgan Rice

In A GRANT OF ARMS (Book #8 in the Sorcerer's Ring), Thor is caught between titanic forces of good and evil, as Andronicus and Rafi use all of their dark sorcery to attempt to crush Thor s identity and take control of his very soul. Under their spell, Thor will have to battle a greater fight than he has ever known, as he struggles to cast off his father and free himself from their chains. But it may already be too late.

Gwendolyn, with Alistair, Steffen and Aberthol, ventures  deep into the Netherworld, on her quest to find Argon and free him from his magical trap. She sees him as the only hope to save Thor and to save the Ring, but the Netherworld is vast and treacherous, and even finding Argon may be a lost cause.