Raymond E. Feist : Shadow of a Dark Queen - Book One of the Serpentwar Saga

A dread darkness is descending upon a great land called Midkemia—a powerful and malevolent race of monsters that has slipped through a hole in the dimensions. And two unlikely young heroes—a bastard heir denied his birthright and an irrepressible scoundrel with a penchant for thievery—must take up arms in the struggle to protect their besieged world...two friends chosen by Destiny to stand at the fore of the battle that is to come against the gargantuan reptile army of the terrible Emerald Queen.