Dark Is the Sun — Philip Jose Farmer

Fifteen billion years from now, Earth is a dying planet its skies darkened by the ashes of burned-out galaxies, its molten core long cooled.

But young Deyv of the Turtle Tribe knew nothing of his world's history or its fate. He lived only to track down the wretched Yawtl, who had stolen his precious Soul Egg. Together with Vana, a girl from another tribe, and the plant-man Sloosh--both also victims of the same thief--they trailed the thief across a nightmare landscape of monster-haunted jungle and wetland.

The search for the Soul Eggs led the troupe into deeper and deeper peril -- first to the lair of Feersh the Blind, the witch who had ordered the thefts; then to the Bright Abomination, the jeweled wasteland that harbored The Shemibob, the ageless being from another star who knew Earth's end was near...and held the key to the only way for any to escape that end.