Dark Ascensions by Seymour Benson
Bendahrin lived an uncomplicated life as a collector of rare antiquities. On a whim, and for no reason that he could ever remember, he decided to invite the most powerful, evil sorceress in the galaxy to meet him on her home planet so he could attempt to steal her legendary sword, Bratenla. Instead of killing him outright, in an unexpected and unprecedented act of mercy, she, Alystra, the Destroyer, rescued him. He suddenly found himself trapped on her ship, chasing across the galaxy, helping her to search for and discover the source of the spell that had transformed her into the embodiment of evil. Life with Alystra, for Bendahrin, became tremendously complicated, and potentially deadly.