Tad Williams 
Shadowrise 
Shadowmarch Book 3

As shadows threaten to consume the kingdom of Southmarch, Barrick Eddon, heir to March throne, battles his way across the sinister Shadowlands. He must journey through this dangerous, inhuman realm to fulfil a pact – as this may be all that can prevent the atrocities of a full-scale war with the Twilight people of Qul-na-Qar. Meanwhile, the assault upon Southmarch has truly begun. Yasammez, the formidable head of the Qar army, has ordered the attack, believing that the pact between humans and Qar has been broken. Unless Ferras Vansen, Captain of the Southmarch Royal Guard, can convince her otherwise, the humans are sure to meet the dark end that has been promised to them . . .