I am working on my own setting (and might publish it this time...). I already posted two NPCs from it.
So, here's what I want from said setting:
A place where weapons and emotions are the only magic.
That's the setting I'm working on...dual-statted for Low Fantasy RPG and Mythras (I've been toying with the idea of making it tripple-statted for Cepheus/Traveller, and/or one of The Riddle of Steel clones, too - but neither is part of it yet).
Why, you ask me? It resolves "the supremacy of spellcasters" and the complaints about "how can I do a monk and why do I need a weapon" in one fell swoop. And I just like weapons being magic.
Since there's no magic, and a person can only be so strong and skilled, people are your only real, nigh-infinite resource for achieving greatness. You can't defeat an army...but you can lead your own army. Or you can work the army's strategist into committing a mistake, and defeating himself.
And the only way to control people, as we all know, is to appeal to their emotions. (Of course, that means finding out what they want, first. And catering to people's emotions is always taxing).
And some of the people you meet are weapons with spirits inside. Others are demons. Both can give you great power...at a cost, because they also have emotions. Granted, not all of them have the emotions you'd consider sane, but then that applies to those people of flesh, blood and human parents, too...
Swords (and other weapons)
It's important to note that the Spirit Weapons aren't always swords. Sometimes, they're something else, but always have steel parts (making shields and iron-shod staffs eligible).
One thing is for certain: they're virtually indestructible, and never need sharpening (nor is it possible, even if they're less sharp than you wanted them to be - as might be the case with some anti-armour implements, if you ever get to a place where people walk around mostly naked). You never clean rust, nor does it impact the functioning of the weapon, if it's there (on some rare* cases).
Last note: a true demon can never become a weapon. Only once-godlings can, and they can only be weapons. Because weapons are holy.
Don't hope to have both, though. Neither of them are into sharing, and even less are into sharing with another kind. To find two different ones that don't mind, is so unlikely as to be virtually impossible.
The POWERZ of thy sword!
First and most obvious: they give you bonuses and penalties on some Passions. And they demand that you indulge in those that they give you bonuses for, and avoid those where you have penalties...at least they don't make it harder for you to obey. And obeying makes you happy (that's listed here, because a naturally violent person who gets another shot of magic weapon-inspired violence might become a murder machine with no special powerZ needed - but you get those anyway). Keep in mind, it's never just "violence+20%", it's "violence against equals", "violence against groups", and so on. It's never "Love+20%", it's "love for your lawful wife+20%", or "love all the nice women+20%".
The next thing you get...depends on the weapon's personality.
A weapon that can be any other weapon? A weapon that can fly to your hand? A weapon that can help you teleport, as long as you already are wielding it? A weapon that sheathes itself in flame, or ice, or spits lightining or acid in the enemy's face? An weapon that gives you wings? An weapon that executes your wishes, but at a price?
The important thing is, they all have an...call it "theme". That's because they all have spirits inside, dontchaknow, and spirits have...well, personalities.
And all NPC personalities have a "theme", even if the "theme" is "person living in the Wild West and driving cattle around". Or, for a fantasy game, "a normal Nemedian guy who hopes his skill at arms would help him get ahead in life".
Or, in the case of weapons, The Noble Eagle, the Principle of Cutting, the Principle of Insertion, and The One Who Spots Everything (beware of ever meeting The Ruthless Ferret, though - he kills for fear, profit, pleasure, glory, infamy, food, because someone was in reach and his head was conveniently situated to shear off, and because once you start, you might as well go for the full round...)
The Noble Eagle has the powers of Sight (including through objects), See Magic, Flight, Paralysis, Telekinesis** and Longevity. He wants riches, power, but be ready for you your sex drive to take a plunge: eagles fuck once a year, after all, and he finds it distracting.
Farseer also gives you Sight, but also DarkSight, See Magic, and only then Flight (to get to better vantage points), and Longevity. He's quite close to The Noble Eagle, but while he respects the Eagle, the Eagle secretly envies Farseer. His desires are Curiosity, Respect, and Sex (being able to see people naked either makes you horny, or jaded, but being jaded doesn't match well with his curiosity). However, he lowers your desire for mass violence: Farseer prizes understanding and showing off the knowledge so obtained, not killing off. He very much wants battles with worthy opponents, though: what's better than figuring out the weakness of a man with no weaknesses?
Cleancutter is probably his stellar opposite, yet the two are, for some reason, close friends. Cleancutter allows you Power Cuts, Transformation (of Cleancutter, not yourself - you can always carry him like a knife, if only a knife is allowed, or as a glaive, if you face a horseman), Spirit-cutting, Wounding, and allows you to sever causality to get what you want by merely making a symbolic cut in the air. However, something is going to happen, as a price: when you cut the strings of causality, expect backlash! (Also: karma is a bitch).
You get the idea.
*Rusty weapons are alternately said to have spots on their souls, or to be weeping...forever. Or both.
**You suck being an eagle if you can't carry your prey off, if it's struggling, and eagles are long-lived.
P.S.: Those are the versions of Farseer and Cleancutter that I'm using. They might well be different in your campaign.