Death's Master (Flat Earth Series, 2)
The second book in Tanith Lee's classic, award-winning Tales from the Flat Earth high fantasy series.
A long time ago when the Earth was Flat, beautiful indifferent Gods lived in the airy Upperearth realm above, curious passionate demons lived in the exotic Underearth realm below, and mortals were relegated to exist in the middle. Uhlume, Lord of Death, second of the Lords of Darkness, King of Shadow and Pallor, makes an unusual bargain which sets in motion an intricate sequence of events that entangle men and gods, queens and kings, sorcerers and witches, and lowly wanderers.
But when the secret to immortality falls into human hands, dark magic and wickedness are unleashed, testing the bounds of mortal love and sanity, and questioning the nature and purpose of life itself.