Down among the dead men - Simon R. Green

There is a part of the Forest where it is always night, where the tall trees bow together to shut out the light. Men call it the Darkwood.
The border fort on the edge of the Darkwood had been built to keep the peace. So far it had, but after delivery of a large consignment of gold it falls silent, answering neither natural nor supernatural communication.
When the Forest King sends a small force, combining sorcerous and military skills, they find a deserted fortress soaked in blood. Magic reveals something malevolent and unreachable somewhere in the fort. But Duncan MacNeil and his company have human foes, greedy for gold, to deal with before they can take on the inhuman . . .