Hugo Award 1981.
For 150 years, the Snow
Queen has ruled the ocean planet Tiamat, whose twin suns circle the
Stargate that links her world with the seven worlds of the Hegemony. The
Hegemony's technological gifts have made the Winter clans wealthy,
raising them above the tech-hating, fish-farming Summer clans. In
return, Tiamat offers voyagers a convenient stopover between worlds. But
more importantly, it possesses a unique commodity - an elixir of youth
derived from the blood of sea creatures called mers.
Now everything is about to change, for the Stargate is closing, the offworlders are leaving, and the rule of the Winter folk will give way to that of the primitive Summers, who will sacrifice the Snow Queen to the sea and crown a Summer Queen. But the Snow Queen has planned against this event: she has secretly cloned an heir to insure that her strength and spirit will live on...
In the Summer lands, young Moon Dawntreader is in training to become a sybil in the service of the sea goddess. She has no doubts or fears about the future; she only looks forward to the fulfillment of a dream. How could she know that the dream that will shape her life if not her own?