[Mare of Earth]

I read a book around 20 years ago, and have not been able to identify it so I can re-read it. Maybe someone here knows it - I have tried other places and no luck.
Third book in a trilogy set on a world with just islands - I believe the setting was referred to as the Archipelago.

The part that haunts me is a "prophesy" referred to that goes something like:
Circle's center at it's edge,
Serpents head upon it's tail.
Evil's heir in chains set free,
Will chain the sun to break the link.

The general plot is about a massive spiral fogbank consuming the islands and killing\sucking the life out of them - male lead is the prince of the islands, who lives in a castle in the eye of the fog, while the female lead is from a tiny island outside the edge of the spiral.

I have been trying to identify this book for over 20 years - any ideas would be awesome.