“You told us last Halloween that a willing sacrifice would make a far more powerful key than an unwilling one,” Eleanor said. “So here we are. Willingly. And you’re going to finally get what you want.”

“Are you sure about this?” Pip muttered at her shoulder.

She wasn’t—but it was too late to back out now. “We’re the key. The three of us. If we’re willing,” she said, looking at each of them in turn. Otto nodded, though his eyes were frightened. Pip gritted her teeth and dipped her chin once. They trusted her. Eleanor could only hope they were right to.

“A willing key is the most powerful kind of all,” Katie said. She stretched out a hand toward them.

Power flared, invisible strands of magic wrapping around them. Otto and Pip gasped, and so did Eleanor—her skin prickled and stung, like there was electricity dancing over it.

“This is a trick of some kind,” Mrs. Prosper said, frozen in place.

“No trick,” Katie replied. She reached up, and Eleanor could just barely see the gray glimmer of a thread running all the way to the massive door, wrapped in chains. She looked back at Eleanor. “Ready?”

Eleanor nodded. She gripped Otto and Pip’s hands tightly. Whatever happened now, they were together.

The ferocious storm of magic building on her skin began to rush away. Katie was pulling it. Sending it dancing along the thread to the massive door. Mr. January and Mrs. Prosper turned wonderingly, eagerness and disbelief on their faces.

Mr. January had spent almost two hundred years building to this moment. Two hundred years of tricks and rituals, magic and curses, to make the three of them the perfect, final key. He meant to fling the door open. To let all the armies of the Pallid Kingdom spill out into their world, and turn it as gray and miserable as their home. And now the last key was finally sliding into the lock.

Mr. January had turned children into keys, each of them a fraction of the power he needed to open that door. But he had done his work better than he realized. Not one deal, but two linked the children to the door—two bargains, and three magical trials conquered, and their own will, and their blazing love for each other. Katie didn’t need to transform them into the final key. She needed only a fraction of their power to form it, and she drew it from them painlessly.

The key appeared in the lock, and turned.

Katie didn’t fling the door wide. She let it open just a crack. Just a sliver. Not enough to let the Pallid Kingdom and its armies of the Empty out.

But enough to let two wayward children back in.

The chains rattled. The door eased open.

“Now, Eleanor,” Katie said, her voice strained—she was trembling with the effort of holding the door that wide, and no wider.