Gods of the Well of Souls by 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 - but his contact with her had allowed 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 was now held prisoner by a revenge-mad creature who had surgically and genetically altered her into a bizarre form and made her a slave to powerful narcotics. But the Well could allow nothing to stop the guardians' journey for long. For the Well of Souls was only a machine; it needed the Watchers to handle events and circumstances beyond its sophisticated programming. And so it set plans in motion to jolt its champions back into the game. The only other help Brazil and Chang could expect was from the companions who had been swept along to the Well World - by accident or design. But the newcomers had been caught in a web of intrigue and deceit spun by the ruling council, a group desperate to stop both Brazil and Chang. What none of them could know was that all of them, even the great Well computer, were being manipulated by the Kraang, an entity more ancient than the universe itself, a being that would use the guardians to give it powers far beyond even those of the Well of Souls - a being that would become a living god...

This book is part of the "Watchers at the Well" series.
Here are some other books from this series: