I got an idea.
An isekai appears within a stone age hunter gatherer humanity, hunted down and preyed upon by all manner of fair folk, monsters, beasts, and general nasties.
He's got powers, of course. A deep understanding of math and engineering and science, knowledge of alchemy, nanoha style magic, sorcery, youjo senki magic, and all sorts of other shit.
His first step? Protecting a tribe hunted down by a Dark Fair Folk. Killing it by transmuting a hundred likes of iron and then impaling it.
Then he turns to them, and offers them something. Food. Power. Safety. Security. Strength. Home. Respect. Luxury. All they need to do is to follow him. It won't be easy, it'll be scary, but they won't have to worry about being killed like trash ever again.
Some say this is dangerous. Some say this is risky. Some say this is a trick. Some say that this is their only hope. They put it to a vote. They join.
He introduces metalworking, agriculture, and writing. He uses magic to forcibly burn information into one-man mind, and then use him as a teacher. He summons demons, biomechanical constructs, and elementals to forge tons of metal equipment. He uses sorcerous workings to bypass years of selective breeding and genetic engineering, making his own custom plants. Plants to provide medicine, fiber, paint, meat, food, milk, lumber, wrappings, bottles, perfume, lights, waste disposal.... all made.
The first city was built. Then it was attacked. A band of orks, feeling that the pink ones were getting too uppity. They were beaten back. Aura enhanced men clad in plate armour with strength and durability enhancing runes, armed with swords powered with elemental powers, supported by flying mages raining down artillery spells and a radio system. The orks are crushed, and the stragglers sent fleeing. More people come, and the city grows and celebrates, it's population boosted by artificial wombs, modern medicine, working sanitation, and fertility drugs.
This happens again, again, and again. Monsters and beasts attack, are beaten back, more people join, and the city expands. Schools, training barracks, and universities are built, and armies of men are sent out to fight. The flying bird men of omelas are sent crashing from the sky by anti air lascannon and aerial battalions. The iron hard spires of the steel ants were torn down into rubble by the flying battleships of the humanity navy. The underground cities of the mole men were flooded out with thousands of tons of explosives and molten iron. The dream worlds of the fae were destroyed by men who had imagination ripped from them and armed with cold iron.
The war over, the isekai turns to within. Arts and crafts flourish. Consumer goods were made in great factories. Crops were not bred for nutrition, but for taste. If you want to imagine it, think going from stone age Neolithic to 1950 America. In less than 50 years. For the descendants of the tribe, it is like a dream. For the others, too. Of course, it was not perfect.
The isekai was a nice guy. So nice, that he delegated too much. It was sudden. It was said. And he was ousted from power. And he left. His last words were:
"Be well."
The master gone, the usurpers installed a puppet figurehead while they grabbed power for themselves. Money and cash meant for medicine and infrastructure and the public was redirected into private purses. Unrest and conflict was present. Before, the boss had unmatched power and legitimacy. But now... factions within the government, within the military, within the mage schools and within the industries, battled and duked out in shadowed rooms and back alleys. Tempers frayed and shots were fired.
Then, civil war.
Think of it as a combination of the thirty years war, world war 2, world war 1, and the Vietnam war put together. It was long. It was horrific. It was gruesome. The magic and sorcery previously used to preserve human life and free it from the shackles of bondage now ruined it. Razor winds that were powered by ambient mana moved around the wasteland where once stood fields of crops. Silent villages and towns where every living thing was slain by the blue rinse, the only thing left of their former inhabitants now being clothes and piles of dust. Self replicating biological war machines wander the world, serving as hazards and terrors to the survivors. The shining cities of humanity are gutted, sending them down to the level of the dark ages. Once they had plans to fly through the star. Now they struggled to keep the lights on.
The isekai? He disappeared. And in these troubled times, has been deified. Some say his leaving was because mankind was sinful, and will return to bring back the golden age when they have repented. Some say that he left and left them to destroy themselves, because he realized that we were not worth saving, and have decided to finish the job. Some say that he left to test mankind. A test they failed, and so the only way to atone is to restore the glories of the golden age. Some say that he was a demon in disguise, who handed humans the weapons to bring about their own demise. Each sect despises the other. All are very violent.
Of course, you are now the new isekai coincidentally, you are a perfect copy of the previous messiah. Your job is to repair a world ruined by humans and magic. To make sure they no longer self destruct and accept your rule, but not becoming a tyrant nor figurehead.
Have fun.