Track a kill list. Once a weapon takes a hundred souls it gains some sort of power based on the lives it's taken. A weapon that's killed many orcs would gain a preference for them, while a weapon that's recklessly used against enemies and innocents alike may drive the wielder into a bloodthirsty rage.

Throw in some foes who are slaughtering innocents just to power up their sword in one of the early adventures while you're at it. That'll let you introduce the idea within the context of the setting and show them the consequences of abuse firsthand.

Number of kills need not be the only factor, though. You can let them gain power based on valiant actions on their part or powerful foes they defeat. A sword dipped in dragon blood may come eternally aflame, while the paladin's sword, after being lead by his hand to deflect many blows for innocent lives, has begun to do it of its own accord. I'd encourage you to base it on actions on their part rather than simply letting them choose, as it reinforces the impact those actions have.

If you throw it on the player, there's a good chance they're going to try to make something game breaking. The best thing to do is keep the respective character in mind. You said the growths depend on their actions, right? You can expound on this by also basing gains on their personality too.
As for how often the gains should be awarded, that could vary with participation. Or, you could have them meditate/pray to get a new gain after a notable event. Like, a Paladin who stopped an assassination would gradually gain higher range and magnitude to his ability to sense evil intentions.
You could come up with a chart to make things easier, but there's no surefire way of accounting for every possible combination of actions your players are going to take.
This sounds like a really neat idea, though. The players are essentially forging their own weapons through their very being.

>>44243590
If you're going down this route (which is cool) you should probably have it so that everything can be explained to the player as to why the weapon is a certain way (at least to a degree). If a weapon is possessed with a desire to constantly shed the blood of any and all nearby, it should be relatively clear to all players that there is a reason for that (likely that it was used on anybody that moved for a long time). This isn't to say you can't have some "mystery weapons," but they should be extremely rare or be plot devices.

Some abilities could be a bow whose arrows fly almost silently and appear to almost seek towards a targets vitals, as it was used by an assassin for an extremely long time.

A shield can be naturally fire-proof as it was used by a famous dragon fighter, and be significantly more dense than it normally would be.

An axe used by an orc-slayer could speed up the attacks of its wielder in the presence of orcs, at the cost of exerting a considerable willpower to make the wielder engage in combat and then be unable to disengage from that combat.

A sword used by a necromancer could begin corrupting people into undead, with the speed and strength of the disease dependent on how long the sword was used to do so. It could also halt aging and grant the wielder considerable strength, but atrophy his appearance and drain the health of those around it.

>>44243733
for a clearer mechanical approach, you could use this for stages:

Stage 1:100 souls. Once a weapon reaches this milestone it becomes +1 and gains a minor advantage/disadvantage based on the way it was used and enemies it has slain.

Stage 2: 6 Powerful foes. These should be something more than human. The weapon grows to +2 and gains new powers from the lives it has taken.

Stage 3: The power of a hero. The character has completed a major goal or quest arc, imbuing the weapon with the essence of their victory. It becomes +3 and gains new powers reflecting their actions on the quest, as well as a name and renown. People begin to recognize the weapon from stories they have heard. Particularly dramatic failures can have the inverse effect in this stage, imbuing the weapon with a nasty curse reflective of the character's failure. Few weapons surpass this stage.

Stage 4: Broken and reforged. Like their wielders, no weapon is immortal, and repairing a weapon that has become legend is no easy task. However, with purpose and perserverence it can be done. Such a weapon becomes stronger than before, growing to +4 and gaining new powers based on how it was broken and the purpose behind its repair. A weapon broken against a dragon's hide may gain dragon slaying capabilities, while a weapon disintegrated by a lich may imbue its wielder with magic resistance.

Stage 5: Truly legendary. This stage is the hardest to reach; the wielder must have completed all or most of their goals and become a legend in their own right. Being crowned King or ascending to godhood are some of the things that can push a weapon to this stage. The weapon becomes +5 and gains powers reflective of its wielder and all they have accomplish. Every child knows its name and story.

Don't hesititate to deviate from these. For instance, a weapon that has taken no lives may instead gain powers for sparing 100 souls.

>>44243845
I don't want them to find that many weapons, and my idea is actually to keep the weapons worse than their own, with drawbacks they won't accept, or as a type of weapon they aren't even proficient with. I have had a bad habit of primarily giving them swords, which is means my players gravitate towards them. This would allow them to pick up whatever the fuck they want, and still have their preferred weapon be not only viable, but the best option, as it should be. I've had someone switch from an axe to a sword, because the sword was so much better, that despite his being really good at using his axe, the sword was still straight up better. It was a result of my bad GMing and the players not making it clear that I was limiting their choices. I want them to be at their strongest with THEIR weapon. Let them personalise it, give it a name, let people recognise the weapon even when they aren't present themselves. That kind of thing.

>>44243825
My idea of "throwing it on them" was not an invitation to power gaming. It was just a thought of letting them choose.

The idea was to ask "what do you want, and why does it make sense", and then I could just slap down ideas that are too dumb or overpowered, or straight up doesn't make sense.

It might make more sense to only listen partially to their wishes though, since I don't get the feeling that most magical artifacts had their powers chosen by the wielder.

Since we usually go from level 1 to 20+, they will approach the level of demi gods at some point, so I don't want to give them anything too "bad", to avoid them just ditching their weapon because they screwed it over. On the flip side, it would be kinda neat to let them make a few mistakes at first, and them let them commune with the sword, and "give up" on a few of the abilities if they find them too bothersome.

>>44243988
These are fantastic ideas. Especially the reforging part. It would also allow me to pretty much base some of the plots around needing a certain weapon, but having them find the weapon no longer exists, or perhaps never did. Fusing it with spirits or some shit, maybe even allowing them to make Thr weapons empathic, would really drive home how important the weapon has become, to the point of not just being a weapon they have given a name, but a weapon that has been given life because of how much it means to the character.

Would also introduce far more personal plots than I usually do.

>>44243590
Well a pally weapon that either starts out mad strong, but gains a certain number of flaws if the player deviates from their oath/path or something that does that in reverse, giving more power based on good deeds. Self sacrifice means more damage when you are actively protecting somebody, repel enemies if the player has been pushing enemies during combat etc.

Rogue he picks pockets and locks a lot has gains the ability to substitute his tools with his daggers/arrows/bolts, somebody who prefer assassination gets a, well, blink dagger, but only after several successful assassinations where he "vanished into thin air".

Mages who prefer certain types of spells imbuing their weapons with the said magic's residue, like a staff that starts working as a lightning rod of the wizard only throws lightning based spells, maybe becoming permanently lit if he casts light a lot, becoming impossible to follow with the naked eye if the wizard blasts out illusion spells on a regular basis.

Ranger perhaps with a bow that eventually starts growing arrows naturally, the bow giving out buffs to his pets if he's going beast master type, barely visible arrows if he prefers sneaking and so forth.

These are just my two cents on this.

>>44244162
>blink dagger, but only after several successful assassinations where he "vanished into thin air".
We were once given a dagger with the ability to blink up to 500 ft., or to a single predefined location within 10 miles, but only after killing someone, but with no other limitations.

It culminated in a particularly epic encounter where he gained a ton of buffs, and killed 4 people in different parts of the room with his 4 attacks, because he blinked straight to them after each kill, and then vanished.

I can vouch for how awesome that thing can be. If you want to limit it somehow, you could give it a cooldown period, or perhaps just require a specific point of arrival.

For archers, if he keeps recovering his arrows, allow the bow the ability to slow levitate them back towards the bow if they are still in one piece, allowing retrieval even if the arrows are out of sight, or dropped off a cliff or something similar

It was mentioned that this was to apply to weapons, but Shields would make a good addition to that. Some fighters rely almost more on their shield than their actual weapon. Having that gain all kinds of cool bonuses would be cool as well.

Though you probably have to consider how that works for dual wielders as well. Making the bonus apply to only one weapon would be best, balance wise, and a lot easier to keep track of. But it would make characters focused on 1 weapon more efficient

Biggest piece of advice I can give for this is pretty simple.

Players don't need levels as much if they're getting cool gear, the opposite also holds true.

Cool idea though, I had a game where the shattered pieces of the sword Ragnarok got forged into new blades. They did an additional 3d6 damage (1 fire, 1 cold, 1 electric) but had the nasty effect of draining 1,000 XP upon wielding. After 1 minute the drain happened again after a failed Will save (it was 3.5). They also levelled with you, doubling the d6s every 5 levels.

It was a godslaying campaign. Don't judge.

>>44243590
look at erathdawns magic weapons.
they are artifacts of old legends that grow stronger as the wearer fulfills their destiny. complete with ready background writeups and plothooks.
really nice done.

>>44244269
>Players don't need levels as much if they're getting cool gear, the opposite also holds true.
Hey, letting the weapons level up serves this purpose perfectly.

I think I'll go with the grade system, letting it advance on certain triggers, but I'll probably split it up further, like having 3 minor effects, and then a major. So it starts off getting +1 and some major bonus (like a bonus d4 against Greenskins), and then it gets 3 minor powers before it can grow to 2+, adding small things like x casts per day of Light on the weapon, x casts per day of cure light wounds, glow in the presence of orks, etc., to make the weapon grow at regular intervals, rather than just getting a few really huge boosts in power throughout a really long game.