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Left-Handed

Personally I don't see any issue with the prowess bonus, unless you were to code it so that it only works on non-left handed characters; from personal experience in HEMA fencing, and as a lefty, right handed swordfighters are often at a disadvantage against lefties because of the stance difference, as lefties inherently have an advantage, as defensive cracks can be exploited that might be solid in a righty vs righty match. Left handed swordsmen often have no choice BUT to train against right handed swordsmen, since only about 10-11% of the population is left handed.

I would suggest adding ambidextrous if you could, as others have suggested. It should also add prowess, but nullify the prowess loss from losing a hand, since a fully ambidextrous person can use either equally; however, it should also COST a lot, and only have a 1% chance of occuring

Maybe also add dubble handed, yes that is a thing

@Feivel Mind if I add you so I can DM you a bit about the coding? The code in your comment looks like it could work, but I'm having a bit of trouble getting it to show the trait in the game now that I started working on it a bit.

Could this be of any help?
faith.religion = { is_in_family = rf_abrahamic }

I would also join in to say, that +3 Prowess seems too much. I would even be fine with +1.

+3 prowess? For nothing? Don't mind me.
Siriusly, the bonus is just to good for the malus

I'd be happy to give this an update making the changes @Elvennoob described, along w/ a few other minimal changes as well. However, I'm not sure how one would go about coding an opinion penalty for the Abrahamic faiths only...

If someone wouldn't mind pointing me in the right direction, I'd be more than happy to put out an update for this.

still works perfectly, still essential

I agree, they saw as being left handed as a bad omen or a kin to that. Also, this may need an update

I would make it a Christian opinion penalty, or maybe at a stretch Abrahamic Group Opinion. (And not clergy only but just for the entire religion)