Gamearth - The Gamearth Trilogy, Book 1

Gamearth: It was supposed to be just another Sunday night fantasy role-playing game for David, Tyrone, Scott, and Melanie. But after years of playing, the game had become so real that all their creations - humans, sorcerers, dragons, ogres, panther-folk, cyclops, etc - now had existences of their own.
And when the four outside players decide to end their game, the characters inside the world of Gamearth; the warriors, scholars, and the few remaining wielders of magic, band together to keep their land from vanishing. Now they must embark on a desperate quest for their own magic - magic that can twist the Rules enough to save them all from the evil that the players created to destroy their entire world.