The empty throne - David M. Allan

Three thrones, metal, wood, stone, stand around an uncloseable gateway to a new world. Hostile incorporeal creatures have tried to come through, but the throne room is guarded. Expeditions have been sent through the gateway, but none has ever returned. On each throne appears an image of one of the heroes who led the expeditions. While they occupy the thrones the gateway remains quiet and there are no incursions. But now the Throne of Stone is empty. An incursion has started, a kulun escaping into the world, capable of possessing and controlling humans. The most powerful families play politics and pursue their own petty aims. Family feuds, border disputes, deep-rooted rivalries and bigotry make for a potentially unstable world, a perfect environment for a kulun looking to create havoc.