Neither of them spoke. The air felt heavy like it did in sacred places, the hush of a cathedral or a graveyard. They followed the compass in silence for what felt like hours, making their way deeper into the belly of the mountain. Ayla couldn’t help but wonder if the compass was broken; it felt like they were just walking in circles. Like they were doomed to be lost in this endless, pitch-black underground labyrinth.

Just as Ayla was about to say something, she realized: it was getting lighter.

The air changed from the dead, stale air of the deep underground to something cleaner, the faintest breeze lifting the hair at Ayla’s nape. As they walked, the cave walls began to glisten with moisture; Ayla ran a finger along the wall and felt dripping water, slick fungus. Soon it was light enough to see the fungus, pale like lichen on the dark rock. And up ahead, she heard—

“Birdsong,” said Crier, breaking the silence for the first time since they’d entered. “Do you hear that? Birdsong.”

“How . . . ?” Ayla started, but didn’t have to wait for an answer. They cleared a bend in the tunnel, and she found out how.

The tunnel had led them to the far end of an enormous cavern. An impossibly vast cavern, like someone had hollowed out an entire mountain, from the foot all the way to the highest peak. The sovereign’s palace and all the gardens and orchards could have fit inside this space with room to spare. Ayla couldn’t even see where the cavern ended, the walls disappearing into a shadowy gloom, but gods, it must have been at least half a mile long. She’d never felt so tiny. Milky sunlight filtered down from high above, and Ayla craned her neck, trying to figure out how far up this place went, but instead found the source of the birdsong: actual birds, turning and wheeling in the light. There must have been an opening at the top, a crack in the mountain like the mouth of a volcano, allowing fresh air and sunlight to stream down.

To illuminate the Iron Heart.

Huge and imposing, almost as tall as the cavern itself, the Heart was a castle. A fortress. A collection of spires. Clinging to the cavern wall like a massive insect, the base was formed of sheer black walls rising from the cavern floor; about halfway up, the fortress joined with the cavern wall, carved directly out of the night-black stone. Hundreds of little chambers and spiraling towers, a city in miniature. Even from here, Ayla could see huge alchemical symbols etched into the sheer walls. The four elements, plus iron and gold.

An underground river cut through the cavern. Green, slow-moving, snakelike, it curved around the base of the Heart. Pale green froth marbled the surface. Ayla and Crier were about fifty paces from the river’s edge—close enough that when Ayla breathed in, she tasted the river water on the back of her tongue, briny and oddly metallic.