“So we can’t use your road?” asked Queen Kol.

“I wouldn’t recommend it,” I said, “the road is much longer than the path I carried you through.”

“You didn’t have to put it that way,” she said.

“And the tunnels are pretty dangerous. There’s a whole bunch of them. They were a pain to make, by the way. And although I don’t think they can cave in because of the way we reinforced them, you never know. I’d be worried about snow or rockfalls blocking you in,” I said.

“We can dig,” she said.

“Most of the roads go pretty high too. Had no choice but to build at ridiculous altitudes. Might be hard to breathe without air magic,” I said.

“Maybe I could cast the magic for a small team,” she said.

“Your magic isn’t strong enough. You’d have to sustain the spell for several days. Only reason humans other than Kelser can do it is because they take turns,” I said.

Frowning, Kol tried to figure out a way for the demons to send a trade or diplomatic delegation through the mountains, but once it became clear that it was too dangerous, and that I wasn’t willing to escort any demons to the double river basin without the permission of the humans, she had no choice but to let it go.

Instead, our discussion shifted to the things we had brought with us. Some dried fruits and preserved vegetables that couldn’t be found on this side of the mountains. Also, the dried and salted meat of the monsters we had herded and tamed, as well as a whole bunch of precious metals like silver and gold. There weren’t a lot of gold deposits on this side of the mountains, apparently, so the small amounts we had brought with us were already quite valuable here. I told Kelser to be careful not to flood the kingdom with gold, since that might wreck their economy. Kol listened to that with a complicated expression, unsure if not giving her people a ton of gold was the kind act I was presenting it to be.

There were other benefits for the demons as well. To celebrate the establishment of the trade route, we decided to build a gift for the demons right in their capital city. Before we arrived, the capital city got its water from a nearby river, with an artificial canal to divert water away from upriver and taking sewage back into the river further down. Thanks to her stay with us in the double river basin, Kol knew that this system was causing a lot of sickness and death in the city, and possibly polluting the drinking water for cities downriver.

“The aquifer to this part of the city can come around here,” I said.

“Isn’t this the fourth one?” said Kelser. “We could probably get away with making a couple.”

“You could also just build filtering systems from the canal instead of doing all this to show off your architectural capabilities,” mused Kol.

“But showing off is important too!” I said with a smile. “That’s what diplomacy is all about, after all. Anyways, there’s a reason for this. Aquifers are much less likely to flood your city if the river swells. Floods will lead to the sort of diseases we’re trying to avoid with this system.”

A team of humans was working on the other aquifers, using earth, water, and air magic to set the concrete and bricks as needed, and using magic hands to guide and form the structure. I’d already taken care of most of the measurements and calculations necessary, so it was a pretty smooth process. The aquifers brought water to four different treatment plants in each corner of the capital city. Here, a simple combination of sand and gravel filters filled water into chambers lined with glass. The chambers were heated by a combination of naturally reflective rocks focusing sunlight into the chambers, and massive wood burning furnaces that would be run every morning to boil the water. Then, the water would pass through a filter made of the charcoal from the fire that had just boiled the water, before going through sand and gravel once again, and settling into storage tanks. Aquifers and small channels would then bring the water to all corners of the city.

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I also taught Kol how to make penicillin. In exchange, I gathered a bunch of plants with anti-septic properties from her kingdom, as well as a few oily plants that provided benefits to things like hair and skin. The demons had already started making soap, since Kol had made that one of her top priorities after her return.

I even found out about a type of disease whose symptoms sounded a lot like smallpox. And just like smallpox, there seemed to be a similar disease in one of the domesticated monsters that the demons kept around on their farms. Excited, I dragged Kelser and Kol to a farm on the outskirts of the city, with Kol’s entourage crying out for their queen and begging for us to slow down. Judging by Kol’s bright grin, I could tell she was glad to be out and about on her own for the first time in a while.

“Is this the monster?” I asked .

“Yes, great elf,” said a tired looking young demon whose clothes were torn and stained in many places. He’d made his clothes even dirtier by falling to his knees when he saw Kol. Couldn’t really blame the guy, though. Why was the queen of the kingdom coming to a tiny, muddy, economically struggling farm like this one?

“Yes, yes, I see!” I said as I crouched next to the monster and stared gleefully at the boils on its skin.

“Is it really that exciting?” asked Kol.

“Oh, absolutely,” I said as I brought out a small metal knife and a piece of glass. I cut open the boil, spread the gross pus on the glass, and covered it up with another piece of glass. The others stared at me without a word. I thanked the farmer for his hospitality, tipped him a lump of gold that made his eyes pop, and dragged Kol and Kelser away once again.

My microscope magic wasn’t good enough to identify this virus, so I had no choice but to run some experiments. First, I confirmed with Kol that the monster virus wasn’t dangerous to demons. Then, I asked if there were any records of who died of different diseases in the city, but she said there were none. In fact, there weren’t a lot of records of any kind, since barely anybody knew how to read or write, and scrolls were mostly meant for the use of the royal family. I advised her to start building up a competent bureaucracy, and to keep records as meticulously as possible. We’d shared with her the technique for making paper, after all, so she could leave the limitations of monster hide scrolls behind!

For the time being, I gathered up some paper and ink, and started keeping my own records. I confirmed that the demons most likely to get and die of this smallpox-like virus were young, and that there were no farmers or butchers in the list of patients and deceased that I gathered by interviewing people all over the city. It wasn’t an exhaustive survey, but it was good enough for me to proceed to the next stage. Demon trials!

I used small samples of the monster virus to inoculate volunteers around the city. With the queen’s support, and my promises of gold rewards, many people were lining up. I even got to include a few children, which was very helpful. The demons had longer lifespans than humans did, so their birth rates were also really low. This meant that any lives I could save would help increase the population several-fold.

It took a few months for the trials to be done. Kelser and the other humans were already taking turns in the demon kingdom with me, although I’d stayed here throughout the whole process. It was only once I was completely sure that this method would not have any side-effects, and was safe for demons of most ages, that I recommended to Kol that she inoculate everybody in her kingdom.

With the invention of vaccines, my knowledge and wisdom of diseases and disease prevention had risen to great heights. Someday, I might be able to control microorganisms through magic, directly. But for now, I was satisfied that I had saved so many demon lives.

Why had I done that, you may ask?

I faced East as the sun began to set on a beautiful spring evening. In the distance, lay my next target. Unlike the first time I had faced this enemy, this time, I would be prepared. I would have two species behind me. Magic at a level I could never have dreamed of before. Weapons, inventions, everything that had leapfrogged the natural development of this world by a far larger amount of time than the time that I had lost during one fateful, maddening conversation.