“I’m hoping you just … know.”

“The scientific method.”

His jaws opened, he was about to say something, probably something like “Precisely”, but he didn’t. He stopped. China lay very still in the corridor ahead of them.

Skulduggery sprinted to her. Valkyrie hobbled as fast she was able, reaching them just as China’s eyes fluttered open. “I’m fine,” she said, her voice a whisper. “Darquesse?”

“It’s over,” Skulduggery said, her head in his hands.

A slight smile. “Not yet.” Her blue eyes flickered behind them. “Ravel. Trouble.”

She dipped into unconciousness, and Skulduggery left her there and stood. Valkyrie followed him into the Accelerator Room.

The Accelerator itself was churning. The humming sound was coming from deep within its core. And it was getting louder.

The Engineer stood looking at them. Ravel was slumped on the ground, his hands shackled behind his back. He was conscious but clearly dazed.

“What happened?” Skulduggery said.

“Erskine Ravel demanded that his power be boosted by the Accelerator,” the Engineer responded. “While I knew that the title of Grand Mage now rested with China Sorrows, I had never been given the specific instruction to disregard any orders from Mr Ravel.”

Valkyrie glared. “So you said yes?”

“I am a robot,” said the Engineer. “I obey my instructions to the letter. There is no room for personal interpretation.”

“And did it work?” Skulduggery asked.

“Yes. But when Mr Ravel stepped out he was somewhat disorientated. Upon her arrival, Grand Mage Sorrows engaged him in what was a quite spectacular physical confrontation, during which she was able to secure these shackles upon his person.”

Valkyrie frowned at the Accelerator. “It’s overloading.”

“I am afraid so,” said the Engineer. “I warned Mr Ravel that his usage would hasten the countdown towards its end. He chose to ignore me.”

“So how long do we have?”

“Twelve minutes, eleven seconds.”

Valkyrie paled.

The circular platform within the Accelerator, the dais, lit up.

“One soul,” the Engineer said, “willingly given. The individual steps on to the dais. Death is instant. Their energy, what my creator called their soul, is then used to deactivate the Accelerator.”

“Well, OK,” said Valkyrie. “OK, so there are loads of injured people upstairs. Loads of dying people. I’m sure we can get one of them to volunteer.”

The Engineer held up one metal finger. “A clarification,” it said. “When the soul is used to deactivate the Accelerator, it is used in its entirety. If there is a heaven or an afterlife, it will not journey onwards. Its energy will not rejoin the Great Stream, if such a thing exists. The soul will be used up, here and now. Never to return.”

“But …” Valkyrie said. “But … wait … Erskine, you did this, so this is your chance to—”

Ravel looked up at them, and shook his head. “I’ve sacrificed enough,” he said. “You’re on your own.”

She looked at Skulduggery. He tilted his head towards her.