Mindbond

In the second book of Nancy Springer’s remarkable Sea King fantasy trilogy, a king’s quest for the survival of his world leads him and his bond brother to a terrifying realm of the dead beneath the water

Once, great kings reigned in mighty, teeming cities. Now six small tribes inhabit the known world—and their world is nearing its end.
 
When Rad Korridun, king of the Seal Kindred Tribe, first encountered Dannoc, the troubled chieftain saw clearly that the man was mad, while Dan thought Kor was the god he had long been seeking. However, the unique connection they shared could not be ignored, and a powerful brotherhood was born. The arrival of Tassida, the beautiful mystery, strengthened that bond and threatened it as well. Now that the beasts of their world have fallen prey to the terrible hunger of the Devourers, the survival of the tribes depends on these three whom Destiny is leading to the undersea land of the dead. There, Dannoc seeks to reunite with his lost father. For Tassida, it is a homecoming. But Kor faces a reunion of a much darker sort, for here lies the source of all the world’s ills.