Saga of the Well World #5: Twilight at the Well of Souls by Jack L. Chalker

This is the way the world will end…The rift in the fabric of space was fast approaching the Well World, and time was running out. Troops all over the planet were gathering for the final battle.
Nathan Brazil and Mavra Chang somehow had to reach the Well of Souls in time to save the universe and before any of the hostile natives managed to kill them.
At best, a difficult mission. At worst, impossible – especially since there was a price on Brazil’s head and many would be claimants! For Brazil, the difficult was but the work of a moment – the impossible would take a little longer!