Fortress of Ice - C. J. Cherryh - Bog

Aewyn's half-brother, Elfwyn Aswydd-the bastard son of the king 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.