A character is instilled a curse that can either give the character vast amount of power, or the opposite. The penalty for taking advantage of that will sap away the character's life, or at least kill the character emotionally.
What I'm looking for is a character who can only speak a fixed number of words per sentence, in exchange of vast fighting abilities. Exceeding those limits will slowly destroy the character's body, and continuously exceeding that limit will kill the character. Due to the curse, the character is antisocial, and uses body language to communicate, but not much.

The second part is what I'm looking for, but some of the first part I can also look through.

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Is this a manga you read and forgot the name of, or is this something you would like to see in a manga and want recomendations?

Sorry if im not getting exactly what u mean by all this.

I'm constantly trying to look for new things in manga, so yeah, I'm looking for recomendations.

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For the first part I think Blood of Yougen kinda fits.

Dragon Hunter - manhwa, not really recommended but it has character that meets your requirement.

Hiei in yuyu hakusho
Variante/Dawn Tsumetai te (sp)/Project Arms - has a common theme
Hanako and the Teller of Allegory tells the story of a folkfore detective which has some supernatural ability.


I dont have anything for the second part but if you like rules like these HunterX has a lot of them.

if your ok with shoujo then Kanata Kara...

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The sweeter the apple, the higher the branch. The quieter the fart, the nastier the smell.
GUESS WHO??

Your requests are very specific, I think it's quite difficult to find something exactly like that, so at least I can suggest Chrno Crusade since the main char has a pact with a demon which uses her life span to be able to fight in battle.

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What I like in Trivial Pursuit style. Pick your category:

I think you have a better chance of drawing a manga like that yourself than actually finding one like that in existance. .__.;;

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♪MONSTARR~ will eat all your cookies and steal your bishies~♪ Φ_Φ

"......"

I'm working on it.

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maybe kurogane its not really a curse, but his power increased and he cant really speak so he communicates with body language (not much). he is antisocial and killed emotionally but no drop in abilities if he talks either though (he cant but thats beside the point)

Not a manga, but if you've got a PS2 I suggest getting Drakengard. Which centers around that kind of curse by making pacts with Mythical creatures.