This idea was based a bit off Konjiki no Wordmaster

The MC is a normal guy, not otaku, who has and avarage life a girlfirend and a circle of good friends

At the ago of 18 he is summoned to a fantasy word to lead the human army against the demons.

In this Fanfic anything that isnt human is demon wich inclue elves dwarves and so on.

When the MC heres why he was summoned he askes if the demons were attacking but his awnser is far from it. The humans want to kill them simply because thwyconsider them a plague on the world as they are unnatural monstrosity created by evil gods.

Upon hereing their reason the MC upright insults all of them and leaves to advanture in the new world he is living in now. He soon discovers that he has a special power to transform into creature he has made any small amount of friendship with.

He thinks of how to use this and decides to go to the so called demon realm where the demon tribes are spread out and ends up saving a small child from on of the tribes (you deside which spieces) he becomes friends with him/her and can ecome the spieces and starts going around learning about the demon tribes.

After some time the humans summon a new hero after many sacrifices because humans have a small man well and the summon requires a lot of mana they get someone that is easier to sucker into the role of heo but ibstead use the typical excuse that demons are attacking them instead of the one that was used for the MC. This gets the nerd roaring for action and the humans soon attack but it ends up attacking the first village the MC visited. The kid he saves is able to escape and heads into the demon relam to find him.

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After a few weeks the kid finds the MC and when he hears of this he gets pissed off and they and the village they were currently in spread out and warned the other tribes.

The MC heads to the village closes to the borders and warns the village just as the human army appears after the tribe had evacuated. They encounter the MC and after seeing a noble who was at his summoning the MC drops his desguise and starts insulting the nobles, the humans and the isiot hero.

The two end up fighting the MC beats the hero because he has fought many monsters between the demon villages by himself.

After the MC beats the idiot hero he feels disgusted being an human and transforms into the race that was first killed by the humans and because of his new hate for humans his transformation ability destroys his human form which makes itimpossible to return to that form.

He escapes while the humans retreat. The humans soon start calling him the demon lord and to retailate he starts uniting the demon tribes and makes a kingdom and starts the war with humans as the demon king or the demons hero

***

Here is a new idea that im sure few have thought of or didnt know how to go by writing it.

I hope to see someone bring this to life or something similar.