So, no shit, there he was. Costan just appeared, naked as a jay bird among a group of what could have been modern Japanese teenagers in some medieval castle throne room. He was only a little taller than them and somehow in what would be the pre teens if he were a human.
Everyone looked at him. He looked down, and exclaimed in Japanese about his lack there of clothing. Nothing like a quick "where are my clothes!", to get people to realize that you are a sentient person. A reply came rather quickly, "at least you aren't another monster like last time."
The "what" was universal. No explanation was given, just a proceeding on to introductions and checking of gods given gifts of the summoned heroes. The king was pleased with everything, except the nudity that was ignored and Costan's gift. The mirror thing that was used to check improperly translated, a few times, his gift as internet grocery, somehow still getting in the ballpark. They decided unanimously that Costan could go.
There were some pretty wacky abilities given to the teens, them dressed up in their school uniforms. Some of them got obviously useful combat abilities while others got support ones. Costan's gift was declared trash, thankfully they didn't see everything else from all of the other times. The king was going to kick him out, but some of the teen girls resisted after realizing that Costan was a very floofy cat man. He still got the boot.
He checked it himself. Online shopping plus, he could buy anything ever sold via the Internet of anywhere so long as it fit within the level and prestige budget. Some old magazine and paper catalog even counted despite being digitally archived and not an actual sales website. His current limit was civilian stuff of no greater value than a silver coin per purchase. This completely redundant capability would level up with him.
He put on some clothes from one of the many inventory spaces on his way out of the castle. They didn't even offer him clothes before kicking him out of the castle. He quickly got some shampoo and conditioner from his storage and sold them at the market, then sold a bunch of other stuff like spices and soaps to the merchants there.
It didn't take him a day to have a couple of contracts for hygiene products and a house lined up. Everyone liked the three in one non irritating shampoo, they liked it a lot. It was surprising at how fast Costan managed to make it big in a nation of racists and bigots, just got to have something that they really want.
Everyone wanted the new soaps and stuff, so he had it made. Then nonsense started with some lady pretending to have had the sole rights to him and his things. It ended up going to court in front of the king. The court used some fancy candles that went out when someone lied.
The king showed his corrupt nature by declaring her as Costan's rightful owner despite the candles and a lot of evidence showing that she was lying. The court was shocked at the ruling. Then they were doubly shocked when Costan called him out on it.
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The candles didn't go out as Costan ranted. "You summoned me to this world. Then you kicked me out of the heroes party without clothing or money. When I started selling things, this covetous bitch claimed to be my owner, when she was not. Now you're declaring me her slave, you corrupt cock gobbling bastard? You must want war with my home empire, but you will not survive it."
With an ignorant retort of "Are you threatening me slave", King Too Unimportant to Remembers Name sealed his fate and that of his nation. Costan couldn't help but aliken the king with the curly blonde hair, sporting a blue chainmail cowl under his crown, to the cartoon character Beavis. He opened one of his inventory spaces and walked in chuckling at thought of King Cornholio of the Bungholes. "Are you threatening me?! MmHmHmhe!"
Everyone was shocked that "the beast man" not only threatened the king, but laughed at him and left. They were way more surprised when he came back clad in obviously magical armor and with many soldiers. The mood quickly went from "how scandalous" to "oh no". Costan walked to the nearby balcony and, with a wave of his hand, opened what appeared to be many large portals above and throughout the entire kingdom.
Costan gave one statement, "Surrender or be destroyed." The king offered another insult and accused him of being with the demon king. Costan responded by drawing a sword and bisecting the king, his throne, and part of the castle.
It was a very quick conquest, lasting only thirty minutes. A force with superior numbers and everything else versus a medieval magic force; he steam rolled them. Costan quickly set them up with a parliament and a better government, getting rid of slavery and bettering conditions. Other countries thought it would be prudent to invade on the coat tail of a successful invasion, but they got rolled up like carpets as well.
Costan wanted to stick around and play shop for a little while, but he never gets what he actually wants. It's almost like the Universe loves screwing with Cosin Tangent. After the full efforts of the surrounding monarchies to claim something for themselves, Costan made them all one big nation. Dissidents got sent to one of Costan's agricultural planets for reeducation, or rather, to be worked like rental equipment until they stopped opposing the new government of the New United Nations of Ekraka. Stupid names for stupid worlds.
Then he went on to find the "heroes" and "aquire" their "help" in defeating the big bad guy. He just flew out to BFE in an exoatmospheric troop shuttle where they were and pulled the classic "get in dumbass". It was a matter of single digit minutes for the "demon kingdom" to surrender when the space navy showed up and vaporized the military installations.
The "heroes", who had been struggling with leveling up to take on the demon army, were shocked. They called Costan a monster, but he corrected them. "I am Costan, the Emperor of multiple interdimensional nations under my banner. This is simply what happens when someone pisses me off. And no, this isn't part of the stupid skill that I got when I was summoned along side of you."
Everything was downhill from there. The kids got to go home and Costan eradicated every trace of slavery. He ruled over the planet for a few hundred years before someone found a poison that could actually kill him and everyone died from it. The gods of that world were angry, but it was one of their followers that unleashed the apocalypse.