The Monster in the Hollows, by Andrew Peterson.
After a long trip across the Dark Sea of Darkness, and the harrowing experiences with the Fangs of Dang, forest-roaming toothy cows, a sea serpent, child slavery in the Fork Factory, and the battle of Kimera, the Igiby family arrives off the shores of Anneira, Nia Igiby’s homeland and its queen. Janner and Kalmer (her sons), Leeli (her daughter), and Podo Helmer (her father and former pirate) arrive with her. Janner had dreamed of living again in Castle Rysen where he was born. But his dreams turn to foolishness. Gnag the Nameless set it afire nine years prior, and it is still burning.
Janner was tired of running. He wanted a place to call his own, a place where Fangs didn’t roam, where Stranders didn’t want to cut his throat, and where he and his family could finally be at peace. (p. 9)
The children ask how it is possible that the land was still burning. Nia's answer somberly reveals the gravity of the threat that is pursuing them.
Nia wiped her eyes. When she spoke, Janner heard the tremble of anger in her voice. “Gnag has hate enough in his heart to melt the very foundations of the castle, down to the bones of the isles itself. He won’t rest until Anniera sinks into the sea.”“But why?” Janner asked. “Why does he hate it so much? Who is he, even?”“Who knows? When it rages long enough, hate doesn’t need a reason. It burns for the sake of its own heat and devours whatever, or whomever, is set before it. Before the war, rumor came to us about an evil in the mountains—but Throg is a long way from Anniera. We never imagined it would come to us.” Nia closed her eyes. “By the time we realized the Fangs were after Anniera, it was too late. Your father believed the Symian Strait would protect us—or at least give us time to mount a defense.” She shook her head and looked at the children. “The point is, Gnag seemed to come from nowhere, like a crash of lightning. He wanted Anniera. He wanted us dead.” (p. 9,10)