Obsidian Ridge (novel)

Obsidian Ridge by Jess Lebow is the second book in the The Citadels series.

"This time the castle is the monster!

Obsidian Ridge (fortress) hasn't been seen in Faerûn for hundreds of years. It's a legend, a fairy story–until it appears, silently and without warning over the kingdom of Erlkazar, blotting out the sun. Steered by the madness of a cunning wizard, the citadel and its vast array of shadowy monsters will destroy all of Erlkazar unless the wizard gets what he wants: the princess of Erlkazar as a bride. But he'll have to battle the king's personal assassin, a loyal courtier, a complicated killer, and the princess herself to bring his plan to bear."