Fortress of Ice — C.J. Cherryh

Sixteen years have passed since the dramatic events of Fortress of Dragons, and Cefwyn, king at last, must rebuild his devastated kingdom. The embattled ruler is aided by his powerful friend Tristen Sihhe and two surprising allies in a struggle he must win: his two young sons. Elfwyn Aswydd, the bastard son of Cefwyn and the sorceress Tarien Aswydd, has spent years unaware of his parentage, yet now it is his time to emerge and claim the gifted birthright he's been denied for so long.

But a dark, sinister magic has crept close to the young man and seized hold of the kingdom. Nothing is as it seems, and the bonds of family strain against the powerful forces that would see them undone. It is up to an embattled four-;Elfwyn; his half brother, Aewyn Marhanen; Cefwyn; and Tristen-;to unmask and destroy the dark forces that threaten to unhinge the king's peaceful and fragile reign.