Helliconia Winter

From back cover of Ace paperback May 1987:

Image Helliconia -- a world at the end of a reign that has lasted nearly two thousand years. The glorious civilization that blossomed during spring and summer has faded, and the dismal wasteland of winter is rapidly destroying the human race. For it is the chill end of the Great Year, the 2500 year cycle in which each season gives way to a new oppressor. And now, as winter approaches and the numbers of mankind dwindle, the barbaric phagors prepare to seize their chance to rule again. And they will succeed. Unless mankind can be united to stop them.