When I got to the Engineering section of the train Asser was tinkering with one of the metal horses while the Scholar, Jez Trattles, stood to one side watching him and taking notes. They seemed pleased to see me.

“How did your gear perform?” said Asser.

“The crossbow was excellent. A+. No notes. The Messer exceeded my expectations. Double amputation just below the knee, through armoured greaves.”

Asser blushed at the praise, less like the master craftsman he was and more like he’d been voted the prettiest girl at the fair.

“The bandoleer, though,” I said, “That needs a little improvement. Turns out Cat-kin have terrible colour vision, then Amris got shot in the reading glasses and couldn’t decipher the labels.”

“Is he okay? Did he lose an eye?” said Asser, looking worried.

“Why would…” I started to say before my brain caught up with what I’d said. “No. The glasses were in his waistcoat pocket. He got shot in the lungs. He’s fine.”

“So what I’m hearing,” said Jez, “Is that some kind of raised marking, something you can read by touch, would be a good idea.”

“Military grade potion bottles and vials are all different shapes so you can tell them apart even in the dark,” said Asser. “But we didn’t have the time or the equipment to do that. We could maybe manage something with a hole punch and a tag though.”

“Have a think about it,” I said, “But not right now. I need to talk to you, it’s about magic…” I stopped because I couldn’t remember what I wanted to ask him. I was sure I’d had a clear idea when I came to the workshop but it seemed to have escaped while I was distracted. Now some other idea was demanding my attention. Something about the metal horses.

I tapped one of the metal horses and it gave a dull but clear ring. “When I saw them riding these things I thought they were using Blizzenpaards in this central cage but no matter how fast they went it didn’t seem to get any colder.”

Both Asser and Jez looked positively delighted by the question.

“You were right,” said Asser. “They are using a kind of Blizzenpaard, winged ones, but they’ve done something really very clever with this device.” He flipped the horse over, showing me the underside of the saddle area where there was a crop of shimmering crystals each about the size of a grain of rice.

“Blizzenpaards do normally create an area of extreme cold when they move,” said Jez. “They convert ambient Source into kinetic energy but the conversion has an energy cost. When people first learned magic we would use some of the Source to pay that cost but that’s very inefficient. Now we channel the Source through our bodies and use the energy from our food, or stored in our fat reserves, to pay the conversion cost. That’s part of why magic is so draining. That’s not how Blizzenpaards do it though. They have a trick.”

“The train engine uses the same trick,” said Asser. Jez looked only slightly put out at being interrupted and motioned for him to go on. “Both the Blizzenpaards and the engine draw heat energy from the surrounding air and use that to pay the conversion cost. Much more efficient. In the engine it keeps the turbine blades from overheating.”

“And in the wild,” said Jez, seeing his chance to interrupt the interrupter. “The Blizenpaards use the extreme cold as a defence against predators, particularly source eaters.”

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“So what do the crystals do?” I said, before these two could competitively interrupt each other any more.

“They’re Source accumulators,” said Asser, as if that explained everything.

“I thought no-one used Source accumulators any more?” I said, remembering the image in the illuminated manuscript of the void squid reaching down out of the sky for the wizard with the golden orb. I shivered at the thought of it.

“People mostly don’t,” said Jez. “They’re not very efficient, or stable and they can attract Source eaters. And of course they give off a lot of waste heat.”

Waste heat? Why did that suddenly seem important? Why did my brain fixate on that phrase so strongly?

“Which is exactly why the Ostians are using these crystals,” said Asser. He sounded extremely pleased with the idea. “The crystals are absorbing ambient Source and storing it so there’s a steady power source for the Blizenpaard but they’re also keeping the saddle warm so the Blizenpaard’s cold field doesn’t freeze the rider.”

That reminded me of something. Something to do with the cooling blankets that Master Armstrong had made for the hospital.

Then I felt it. I felt some great, dark, secret thing watching me. It wasn’t there in the workshop with us but only because it was everywhere.

A terrible fear gripped me that I’d felt this before, I’d felt this more than once, maybe many times, and I’d forgotten every single one.

“Stop talking,” I said. I knew that if Asser or Jez said one more thing my mind would be drawn to follow whatever they said and this moment of understanding would be lost. It would disappear to wherever all those other lost memories had gone.

I was close to understanding something huge and this… thing didn’t want me to think about it. It must be something simple. If it was something complicated then the brilliant people like Jez and Asser and Master Armstrong and Gertrude and Trudy and Roly would have solved it by now. If it was something complicated then this dark thing would have nothing to fear from me. I didn’t know enough to crack something complicated.

If it was something simple then these great brains might not notice their thoughts skipping over it, or sliding off it the way thoughts did around the Fever.

Ha! So it was a thought about the Fever then.

Something about the Fever and cold, or heat, or crystals. I could almost feel the shape of it and it was so ridiculously simple that I almost dropped the idea out of sheer embarrassment.

“So when things absorb and accumulate Source they get hot?” I said.

I felt that dark thing turn its full attention on me the moment I spoke. I felt the weight of it on me. I felt its hunger. I felt my thoughts slowing and sliding around as something pushed them away from this very simple idea.

In desperation I reached for something that I wouldn’t be able to forget. I thought about Trudy taking me to the Cryo Room. Then I thought about the light leaving her eyes as she died in front of me.

The weight of her loss was so heavy on me that I barely felt this thing that did not want me to think. The enormity of it pushed this terrible void of memory to the very edges of my mind. I had no fear left for it. Fear was nothing in the face of the knowledge that Trudy was dead and that I was at least partly responsible for it.

Keeping the memory of that moment in mind didn’t leave me a lot of space for thinking about anything else but I could feel other memories bobbing to the surface.

I remembered Cynthia, in her cosy house on the edge of the forest with her huge dog. I remembered her saying, “Of course I came of age during one of the hungry times. The fever hospitals were full, there were armies on the march and Outlanders popping up everywhere.”

I remembered the diary in the house in Rotveil mentioning “hungry people”.

I thought about the testimony of the sailor and all the beasts in the hungry sea clawing at the sides of the ship, desperate to get to the children with the Fever.

I allowed my thoughts to return to the thing that was currently constrained by the weight of grief, and loss, and guilt. I could feel its hunger. It was an empty thing but it was so incomprehensibly vast that its emptiness had pull. Like the vacuum of space drawing all things to it. If the Source was the hole in the world where the magic gets in then this emptiness was its antithesis.

I realised that Asser and Jez were still standing there, silently, waiting for me to explain myself.