"The Incarnations of Immortality" is an eight book series written mostly in the eighties by Piers Anthony. It is his masterpiece, showing the literary world that he can write a truly deep and meaningful fantasy series.
Most of what he'd written before this series began was rather juvenile and shallow, if not derivative, although the "Man and Manta" trilogy from the late sixties/early seventies comes close, it falls short only because the major characters are a bit flat and trite.
"Incarnations" begins with "On a Pale Horse", the story of a man who accidentally kills the incarnation of Death (Thanatos) during a suicide attempt, passing the role of Death on to himself.
Watching him learn the roles and ropes of being Death is the really interesting part, and as he moves through the world of gods and Deities, the reader gets to see a version of our spiritual world which brings the mythologies and religions of many cultures together in a firmament which contains a God (Incarnation of Good) who is supposed to be managing the whole show, but whom no one has heard from in many long centuries.
The second book "Bearing an Hourglass" tells a similar story about a man who is passed an hourglass by a stranger in a park, and is thus called on to live the backward life of Chronos, the incarnation of Time.
I think this is the best of the series. It's not that it goes downhill after this book; it's only that the challenges of composing this book, a story from the point of view of a man who must experience life, and learn to be a deity, in reverse. It's handled with just the right amount of awkward confusion, where it could easily have gone way too far in that direction.
The next four books follow in this same vein, each one introducing another Deity (or set of deities in the case of the three fates), and a normal person who must take on the challenge of learning to work within the (literally) Godless system. This included the role of Satan, the incarnation of Evil in the sixth book.
The seventh book, "And Eternity", the finale of the original series, deals with the missing God, revealed to be yet another position for a mortal to fill.
The bulk of the book centers around the conflict inherent in forcing out the old God in favor of a new one who might actually be responsive to the rest of the pantheon, making the incarnation of Good the only one who doesn't get appointed until the climax of the book.
The tale leaves the reader with a sense of assurance that the many strange unfairnesses in the function of the universe, which have come to light throughout the series, might finally be repaired.
Just a few years ago Anthony picked the series back up for an eighth book, "Under a Velvet Cloak," about Nox, the incarnation of Night. In the previous series this figure had been a mysterious force, with the 7 incarnations being referred to occasionally as the incarnations of Day, suggesting that this one incarnation works the other side.
She had played a significant role in the Seventh book, assisting the other incarnations first to realize that God needed a replacement, then to choose their candidate; she seemed to both more and less powerful than the others, apparently having a hand in everything, but never accomplishing anything directly. The new book finally reveals the mystery of her function and origin.
Many fantasy readers, especially young ones, know Piers Anthony mostly from his immature, punny fantasy series "Xanth," never realizing that this deeply wrought and smartly spiritual tale even exists, which is a shame because none of his other series even come close to its mastery.