The series is written by Tad Williams. His other series are Memory, Sorrow, and Thorn, and Otherland, as well as a number of stand-alone books. All are worth a read and are highly recommended. Most are "fantasy"-based, but Otherland is more "sci-fi."
Shadowmarch is a four-book series, with three out right now and the forth due out by November fo this year. Here's the exerpt from Part One, as pulled from Tad's website:
Shadowmarch: Volume 1
For Generations the misty Shadowline has marked the boundary between the lands of men and the lost northern lands that are the lair of their inhuman enemies, the ageless Qar. But now, after centuries of stability, that boundary line is moving outward, threatening to engulf the northernmost land in which humans still live—the kingdom of Southmarch. A magical darkness is growing—reaching foggy tendrils from beyond the Shadowline, and those unfortunate men caught in the sorcerous mists of the Qar either never return, or are forever changed.
For centuries, the Eddon family has ruled in ancient forbidding Southmarch Castle, guarding the border against the Qar's return, but now this powerful royal line has been dealt a devastating blow. The monarch and head of the family, King Olin, is being held captive in a distant land, and it falls to his inexperienced heirs to lead their people in a time of growing danger and dread.
It is on the youngest Eddons, the twins Barrick and Briony, that the heaviest burdens fall. Crippled Barrick, haunted and sickly for most of his life, and tormented by inexplicable nightmares, must cling to his love of his family and especially of his sister, which may be all that can save him from madness or worse. Briony in turn can only watch helplessly while her brother grows more and more strange, even as she fights with strength she did not know she had to hold onto her family's heritage in the face of secrets and perils that no living mortal could have imagined.
But as the Qar end their isolation of centuries and reach out to destroy humankind, Southmarch is also threatened by more familiar enemies. The Autarch, great, mad emperor of the south, whose ambitions are as wide as the world and whose power matches his ambition, is poised to strike. And other human despots with conquest-dreams of their own, not to mention spies and traitors within the Eddons' own court, attempt to strike down the Eddons in their time of weakness. Then a brutal murder turns the kingdom upside down, and the twins discover they can trust no one—perhaps not even themselves.
Briony and Barrick Eddon, who in such evil times have only each other, may lose even that bond as darkness closes over them. As the Qar's power reaches out across their land, will Southmarch Castle, the only home they've ever known, become in fact what it has long been called—Shadowmarch?