Plane Hell: The Hell's Plane Amulet by Paul Morabito. This is a paperback book.

     The Kingdom's of Auria and Falruff alike, become flooded with an unknown source of demon-breed. The alliance does everything in its power to assemble the greatest warriors from across the lands in hopes of pinpointing the source of these disturbances. Knowing that the powers of evil and the forces of the parallel Planes of Hell are too much for the alliance to handle alone, they take on a new task of acquiring allies. Ordered by the High Counselor of the Kingdom, and led by the Keeper of The Realm, the heroes carry out their assignments with cunning intellectual wit and refined skills of combat. The expertise of brave warriors and the concealed history and talents of a young boy, brings them closer to their goal. They soon realize, however, that the hunt for an ancient artifact becomes the death of them. With confrontations from superior Demon-Lords, their numbers dwindle drastically. Not only do they face unbeatable odds against their enemies, but they also begin to find the weaknesses that dwell deep within themselves.