Learn how to fucking format. Don't sneak rules into the middle of big fluffy paragraphs. It was shit in 3.5e, it was shit in every White Wolf product, and it's shit now.
This should not be hidden, as it's the only important part of the paragraph:
>"For the duration, the wielder gains a bonus to damage on melee attacks equal to the weapon's concordance, and advantage on strength checks."
Clean up your wording. You take way too long to get to the point. Yours:
>The wielder may use this ability an unlimited number of times. However, every time the wielder does, starting with the second time, the wielder must make a charisma saving throw, or be possessed by shadows. The check to resist this starts at a DC of 12, but increases by 2 every time the wielder uses Investiture of Shadows. If the user fails their charisma save to resist the shadows' influence, they are possessed for a number of minutes equal to the DC of the save they failed, minus 10, plus the weapon's concordance at the time.
Mine:
>Every time you use Investiture of Shadows after the first, make a Charisma saving throw at a DC of 10 + (2 * uses past first). Failure means you are possessed for Concordance + (DC-10) minutes.
See how much cleaner and to the point that is?
Quantify how concordance increases and decreases. Give it actual rules, like "+1 Concordance if you use it more times than its current concordance before a long rest, -1 if you don't."
>While possessing the wielder, there is a chance that the shadows may still do essentially what the wielder would have wanted to do anyway, but they could also do any number of things.
What is this based on? Is it "GM is given free license to fuck you over", or is there a table?
Beyond that, it's a shitty cursed weapon. Damage is the single most boring thing you can do with magical weapons. Make it do something interesting instead, or in addition to the damage. It's got a ton of shadow fluff, give the wielder concealment or stealth bonuses.