Kingdoms of the Night — Allan Cole; Chris Bunch

KINGDOMS OF THE NIGHT
Ever since Amalric Antero returned from the legendary Far Kingdoms, bringing with him magic and riches, the people of Orissa believed they lived in a golden age. Even Amalric did not suspect the truth: that dark forces from another world threatened his beloved city.
Then a beautiful stranger arrived claiming to be the great-granddaughter of Janos Greycloak, master-wizard and betrayer of Amalric's youth. And Amalric learned that the idyllic lands he had thought were the Far Kingdoms were not--and that the real Far Kingdoms, the Kingdoms of the Night, were all that stood between civilization and unimaginably evil powers from another world.
But now the strength of the Kingdoms was flagging. Only someone with a stout heart and a legacy of magic flowing through his veins--someone like Amalric Antero--could hope to unravel the secrets of the Old Ones and lock the door between realities once and for all. And so Amalric set out, with little more than his wits and the magic of another Greycloak, to find the Kingdoms of the Night...