Twilight at the Well of Souls The Legacy of Nathan Brazil Jack L. Chalker 1984
Welcome to the Well World: a construct of an ancient defunct race known as the Markovians.
The Well World acts both as the controller of and the gateway to 1560 worlds created by the Markovians at the end of their time. Twilight at the Well of Souls is a direct continuation of the story started in The Return of Nathan Brazil.
The rift in space created in the earlier book is now even larger and threatens to permanently damage the Well of Souls. Mavra Chang and Nathan Brazil race to reach the Well and find a solution before it is too late.
Arrayed against them is a myriad group of sentient beings who will stop at nothing to bring down Nathan Brazil. But Brazil is not that easily gotten rid of, and he has already snuck into the Well World without anyone realizing it.
And he has special allies including Mavra Chang and Gypsy, who has suprising powers himself. Twilight at the Well of Souls is the fifth book set in the Well World universe.