“Then give it back,” demanded Kell, surprising himself with his own urgency. He told himself it would be safest on his person, but in truth, he wanted to hold it, couldn’t shake the sense that if he did, his aching muscles would be soothed and his weak blood strengthened.

She rolled her eyes. “Not this again.”

“Lila, listen to me. You’ve no idea what—”

“Actually,” she cut in, getting to her feet, “I’m starting to get a decent idea of what it can do. If you want it back, tell me the rest.”

“You wouldn’t understand,” said Kell automatically.

“Try me,” she challenged.

Kell squinted at her, this strange girl. Lila Bard did seem to have a way of figuring things out. She was still alive. That said something. And she’d come back for him. He didn’t know why—cutthroats and thieves weren’t usually known for their moral compasses—but he did know that without her, he would be in a far worse state.

“Very well,” said Kell, swinging his legs off the bed. “The stone is from a place known as Black London.”

“You mentioned other Londons,” she said, as if the concept were curious, but not entirely impossible. She didn’t faze easily. “How many are there?”

Kell ran a hand through his auburn hair. It stuck up at odd angles from rain and sleep. “There are four worlds,” he said. “Think of them as different houses built on the same foundation. They have little in common, save for their geography, and the fact that each has a version of this city straddling this river on this island country, and in each, that city is called London.”

“That must be confusing.”

“It isn’t, really, when you live in only one of them and never need think of the others. But as someone who moves between, I use color to keep them straight. Grey London, which is yours. Red London, which is mine. White London, which is Holland’s. And Black London, which is no one’s.”

“And why’s that?”

“Because it fell,” said Kell, rubbing the back of his neck where the pendant cords had snapped. “Lost to darkness. The first thing about magic that you have to understand, Lila, is that it is not inanimate. It is alive. Alive in a different way than you or I, but still very much alive.”

“Is that why it got angry?” she asked. “When I tried to get rid of it?”

Kell frowned. He’d never seen magic that alive.

“Nearly three centuries ago,” he said slowly, working out the math (it seemed further away, the effect of being so long referred to as simply “the past”), “the four worlds were twined together; magic and those who wielded it able to move between them with relative ease through any one of the many sources.”

“Sources?”

“Pools of immense natural power,” explained Kell. “Some small, discreet—a copse of trees in the Far East, a ravine on the Continent—others vast, like your Thames.”

“The Thames?” said Lila with a derisive snort. “A source of magic?”

“Perhaps the greatest source in the world,” said Kell. “Not that you’d know it here, but if you could see it as it is in my London…” Kell trailed off. “As I was saying, the doors between the worlds were open, and the four cities of London intermingled. But even with constant transference, they were not entirely equal in their power. If true magic were a fire, then Black London sat closest to the heat.” By this logic, White London stood second in strength, and Kell knew it must have, though he could not imagine it now. “It was believed that the power there not only ran strong in the blood, but pulsed like a second soul through everything. And at some point, it grew too strong and overthrew its host.

“The world sits in balance,” said Kell, “humanity in one hand, magic in the other. The two exist in every living thing, and in a perfect world, they maintain a kind of harmony, neither exceeding the other. But most worlds are not perfect. In Grey London—your London—humanity grew strong and magic weak. But in Black London, it was the other way around. The people there not only held magic in their bodies, they let magic into their minds, and it took them as its own, burning up their lives to fuel its power. They became vessels, conduits, for its will, and through them, it twisted whim into reality, blurring the lines, breaking them down, creating and destroying and corrupting everything.”

Lila said nothing, only listened and paced.

“It spread like a plague,” continued Kell, “and the other three remaining worlds retreated into themselves and locked their doors to prevent the spread of sickness.” He did not say that it had been Red London’s retreat, its sealing off of itself, that forced the other cities to follow, and left White London pinned between their closed doors and Black London’s seething magic. He did not say that the world caught between was forced to fight the darkness back alone. “With the sources restricted, and the doors locked, the remaining three cities were isolated and began to diverge, each becoming as they are now. But what became of Black London and the rest of its world, we can only guess. Magic requires a living host—it can thrive only where life does, too—so most assume that the plague burned through its hosts and eventually ran out of kindling, leaving only charred remains. None know for sure. Over time, Black London became a ghost story. A fairy tale. Told so many times that some don’t even think it real.”