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.
Nathan Brazil is a starship captain carrying passengers and cargo from planet to planet. Answering a distress call, he (with others) is suddenly transported to the Well World by a hidden gate.
There Nathan Brazil must stop mysterious forces from taking control of the Well World, and through the Well World, the universe. But to do so, he must deal with bizarre transformations which have changed people into centaurs, mermaids and giant insects.
In this strange land, inhabited by these strange transformed creatures, who are his friends, and who are his enemies? And what of his own memories, which seem to have been deeply suppressed?
Midnight at the Well of Souls (Saga of the Well of Souls: Book One)
Jack L. Chalker