Exiles at the Well of Souls by Jack L. Chalker (1978, Book)

In the second book in the saga of the Well World. Two criminals are at war with each other and accidentally venture into a portal and arrive at the Well World. From there they continue their fight and strive for dominance of the Well World itself only to discover that the Well World takes care of itself and calls upon Nathan Brazil to return and take care of this new threat to it's inhabitance.

This book also introduces the character Mavra Chang who becomes a very important of the rest of the books in this series.

Please read Midnight at the Well of Souls before you read this book as the foundation for the entire well world saga is laid out in that book and this one makes a lot more sense if you do.

The series continues in book 3 of the well world saga in Quest for the Well of Souls.