Gods of the Well of Souls — Jack L. Chalker

In its hour of need, the Well World's guardians had been neutralized. Nathan Brazil was shipwrecked on a desert island with a seemingly harmless girl who had caused his mind to be invaded, his will sapped, and his mission forgotten. Mavra Chang had been abducted by a vicious gang of inter-hex drug smugglers and held prisoner by a revenge-mad creature who had surgically and genetically altered her into a bizarre farm and made her a slave to powerful narcotics. But the Well of Souls, sophisticated beyond human understanding, was still a machine; it needed its guardians. And so it set plans in motion to jolt its champions back into the game.

But avert if Brazil or Chang could overcome the formidable obstacles in their path, they could not know that all the players, even the great Well computer, were being manipulated by the Kraang, an entity more ancient than the universe itself. For the Kraang had a game plan that would use the guardians to give it powers far beyond those avert of the Well of Souls -- making it a living god....