For my game, which is your standard fantasy-land D&D setting...

Not to mention the skeleton computer.

>Tunnel under cities using Move Earth or an Earth Elemental
>Force Cage
>Fill it with water
>Wall of Fire/Lava with permanency
>Steam pressure has nowhere to go, heat keeps getting added to the system
>Place Wall of Force under the force cage, shaping the blast upwards
>Wait
>??????
>Profit

>Use Stone Shape to make a series of hollow spheres with caps
>About 2.5 foot in diameter
>Fill them with oil
>Use a Wand of Shrink
>Shrink them down to about 2 inches across
>Each still holds a few gallons of oil
>Use Pitch to secure them together around a vial of Alchemist's Fire (Which will break on impact)
>Congratulations, you now have a very, very efficient and lightweight way to start fires.

>Create a giant windmill with heavy weights
>Reverse Gravity spell on one side
>Permanency?
Infinite power

>Wall of Fire, permanency
>Decanter of Endless water
>Boiler
Infinite power

>Ring Gates
>Create a large sawtooth gear that's straight
>Stick one end through the Ring Gate
>Weld the two ends together with Fabricate
>Let it fall and turn another gear until it wears out
Infinite power

Make it Hitler.

I think op needs to elaborate how "standard" the fantasy world is, political system, tech level, culture etc. make huge difference

I meant medieval with most of the countries ruled by kings.

>Expecting a dungeon the party opens the door.
>Instead they find only a large room with hundreds of rabbits inside cages.
>He's been poly-morphing all of the kingdom's prisoner population.

>astral vortex baristas
How terrifying.

I think he meant ballistias, but this got a smirk out of me.

>arming troops with modern steel forging weapons

How is he making the Electric Arc Furnaces to smelt all that steel under the proper conditions?

Without unnecessary detail, think technology, but also, think organisation.
Stuff we take for granted: Comparatively impersonal and nonceremonial hierarchies as in modern government or corporations, bureaucracy, organising large work processes with written instructions, the scientific method for finding and optimising technical problems, and so on...

Realistically?

You have to make them frighteningly competent at an array of disciplines.

Even amongst the Veeky Forums population the actual knowledge of technical or technological innovations that could be used well in a pre-Rennaisance civilisation is almost zero. Most have only the awareness that such an innovation was made, and if you're lucky, what it replaced. With the exception of a few pieces of knowledge that are instantly applicable with no technical steps required, like boiling surgical instruments before use or not drinking spirits out of pewter, there's a bare handful of people who actually know the required techniques to employ the device, and even fewer with the technological understanding of making the tools to make the tools etc. to make the thing.

Take the gunpowder or trebuchet. Everyone knows what those are. Most people know what they're made of. And yet if they had to make one from scratch, they'd make at best a bodge-job version so underwhelming you might as well have never made it. Mixing good gunpowder takes genuine chemical knowledge and skill, especially in a time when measuring tools were shitty. Likewise with calculating an optimal trebuchet.

So that already has defined a lot of your character. They're not just 'some guy', they are someone with special knowledge far beyond the norm, and with the specific knowledge that allows him to make the tools he needs to make what he wants. Either the sort of da Vinci type who really just learns it for fun and is so eerily smart he can do it casually, or someone who has dedicated a truly enormous part of their life to a completely impractical pursuit - or someone who for some reason views gaining the knowledge and skill to rebuild modern society from any given point is a practical skill.

So there's some starting points. The guy is either a da Vinci, a turbo-sperg hobbyist, or a prepper-type who genuinely expected the world to end and finds that actually it sort of did.

Then you have to figure out how the guy didn't die within a week in a world where he doesn't speak the language or know anyone, anything, or have any resources. How did he stay warm, get fed, not break any laws or get murdered, so on.

The character makes themselves, because what you have is actually an extremely specific set of circumstances, that appear only on first glance to be broad. Run through the how one by one and you produce your character in great detail.

Like; what skills did he have to survive the first week?

How did he convince locals to keep him around?

How did he convince the first group of people to put him in charge, especially since his technical accomplishments frankly won't impress anyone until he has kingdom-grade resources to make really impressive shit?

How did he ascend the ranks until he HAD those resources?

And how, having become a political player, did he survive in a world of quite brutal politics despite standing out like a sore thumb, or how on earth did he learn how to act in a 1088 French dukedom etc.

There was a book called 'lest darkness fall" that was really good at showing the trouble of trying to use modern knowledge in a past setting. I liked it a lot when I read it but havent read it in like 8 years so not sure how it holds up.

youtube.com/watch?v=VTzKIs19eZE

That sounds very negative, though it isn't really intended to put you off. Just showcase that there's a LOT of groundwork before you get to the character quirks.

To pull something out of my arse...

Say he's a modern historian, specialising in the late Age of Sail. A reconstructionist who believes that you can't understand artifacts unless you have one and try using one. He's spent his life of fifty years devoted to building as many Age of sail devices as he possibly can, culminating in building, with enormous funding and a shitload of intern/student manpower over years years, a full-size three-masted barque, using authentic techniques. And of course, because all of these techniques no longer exist in regular shipbuilding, he has had to learn how to build up to them from first principles.

When he lands in 13th century wherever, he immediately claims to be an Ambassador from Prester John's kingdom shipwrecked on his way to the capital, which excuses his strange dress and mannerisms, and his lack of language. Locals see his strange formal dress and the enormous gold device on his arm and assume him to be too wealthy to be their problem so they send him up the political ladder. Eventually he ends up in a royal court, where he impresses them by laying out a dozen improvements to their naval vessels that can be made immediately, and over the next decade implements fifty years worth of naval technological advancement every year, letting the shipbuilding industry catch up to him each time so he has the resources to move on, until he has become utterly indispensable to a king, and that king is far more powerful than ever due to their enormously-advancing navy.

Because being 200 years out of date for the modern world means that you still have a 500 year head start on naval technology - and the knowledge of how to apply it.

And because a king with three-masted barques when his neighbours have leaky cogs is going to make you a goddamned national hero.

I get your point.

But none of the items he uses in that video are available in 1666.

Hell, the fact that you can't pop down to Bunnings for 500 75mm timber screws, 10,000 110mm broadhead nails, and a truck of 2440x35x70 structural pine means every single thing you could build now takes a hundred times more logistics to get done.

Either that, or he's some kind of tech historian who's big on supporting his theories through practical experiments (building the things he researches using tools available to people of the time).

Which kind of puts him in the same category as the turbo-sperg hobbyist. Except that he has a degree and is knows the ins and outs of getting grants (which likely serves him well when he's called upon to do politicking).

Boiler eruptions is serious fucking business, the amount of steam that gets into your lungs which causes you to literally be boiled from the inside. That city is going to be dead with the buildings intact.

Bbeg was a petroleum geologist in our world. He finds himself back in time and survives a few weeks through luck and hardiness of having worked in deserts, tundras, and violent oceans.

He falls in with some poor villagers who let him stay with them as long as he works the fields during the day. After work, our bbeg takes walks throughout the country and gets a lay of the land and sees rock outcrops. Over several months he begins to understand the state of the world he lives in (magic, monsters, preindustrialization) as well as an idea of the local geology (some structural deformation and primarily sedimentary rock).

He begins to spend his evenings building a wooden structure over what he thicks is an anticline a mile or so from the village. Nothing fancy, just a old fashioned drilling derick with a bit fasioned of broken farming tools.

Six months and 4 holes later he strikes gold. Black gold.

3 years later our bbeg is a regular john rockafeller. His village has become a massive hub for his product and he is begining to be able to bring rudementry technology to improve his operations and capture natural gas as well.

Some rival try to emulate his success and while they can easily drill wells, they do not understand his methodology behind choosing where to drill and cannot compete. Those who try anyways are shut down by his growing private army.

10 years in, our bbeg has an oil empire that would make a saudi blush. He scours the continent for the most curious and intelligent thinkers and sets them down avenues of scientific exploration. He is able to use his knowledge of our world to guide them down development paths that will bear the most success. Supporting these people are an army of smiths and laborers. Think edisons invention factory if it was backed by Rockafeller.

Year 20, or TYOOL 3. Our BBEG has total control over society. He has initiated an inquisition to squash out all magic (the only source of knowledge and power that he doesnt have a monopoly on)

Greyhawk is my standard fantasy land.