90 Trillion Fausts by Jack L. Chalker (1999, Paperback)

This is the last book in the trilogy. The teams are looking for a way to defend the demons which can be killed. They seem to have a taste for flesh. There are a large number of them. They were imprisoned for millions of years. This book also answer the question about the origin of the the universe. Some of the answers are very surprising and all of the members of the teams have gone their changes during their journey. They have new mental powers and increased ability in the powers they started with.