The war within - Stephen R Donaldson

It has been twenty years since Prince
Bifalt of Belleger discovered the Last Repository and the sorcerous
knowledge hidden there. At the behest of the repository's magisters, and
in return for the restoration of sorcery to both kingdoms, the realms
of Belleger and Amika ceased generations of war. Their alliance was
sealed with the marriage of Bifalt to Estie, the crown princess of
Amika. But the peace - and their marriage - has been uneasy.

Now
the terrible war that King Bifalt and Queen Estie feared is coming. An
ancient enemy has discovered the location of the Last Repository, and a
mighty horde of dark forces is massing to attack the library and take
the magical knowledge it guards. That horde will slaughter every man,
woman, and child in its path, destroying both Belleger and Amika along
the way.

With their alliance undermined by lingering hostility
and conspiracies threatening, it will take all of the monarchs' strength
and will to inspire their people into defiance...