Shadowmarch, book 1
Tad Williams2004

The frontier called the Shadowline, between the Twilight Lands and those of humans, is being breached. The first Marchlands kingdom in the path of Twilight invaders is in disarray, for its king is a prisoner, and not all accept his elder son's regency. What's more, the cruel empire of the south is moving north. So the Marchlands are caught between two foes while having to deal with internal intrigues and inexperienced rulers. When the prince regent is killed, apparently by one of his closest advisors, the surviving regents are an impetuous princess and a disabled prince. Trust at court and in the kingdom dwindles even as Twilight forces attack, and responsibilities the princess never dreamed of or prepared for fall upon her.