Blade of Languor (cursed) magic item idea

Magic weapon (requires attunemenent)
This weapon has the appearance of a longsword that was once ornate, but has since tarnished and lost all color and shine it may have had. When a potential wielder first touches this weapon while it is unattuned, they hear a myriad of whispering voices offering power in exchange for being taken with the wielder.

Weapon power: Investiture of Shadows
As a bonus action, the wielder may temporarily offer themselves up as a vessel for the envious shadows that haunt this weapon, in exchange for a boost in power. For 1d4+1 rounds (or 30 seconds outside of combat), the wielder's eyes and mouth appear to leak black smoke, as well as any open wounds the wielder might have. The weapon is also wreathed in a similar emanation. For the duration, the wielder gains a bonus to damage on melee attacks equal to the weapon's concordance, and advantage on strength checks. The wielder may occasionally hear the shadows whispering to them. These whispers either sound like incomprehensible mutterings, or jealous mockery of those with physical form.

The first time the wielder uses this ability, the wielder suffers no consequences.
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.

When the wielder takes a long rest, Investiture of Shadows resets back to not requiring a save, until the wielder uses it again.

The shadows like a wielder who uses their power often, and the weapon's concordance will increase based on this. If the wielder neglects to use this power, the concordance may decrease. Increases or decreases of concordance are determined during long rests for this weapon. The weapon's concordance starts at 1 when attuned, and increases to a maximum of 5.

The shadows haunting the Blade of Languor are immune to the effects of Dispel Evil and Good. However, if the wielder is possessed by them, the shadows may be targeted with the Banishment spell, using the wielder's charisma. If successfully banished, the shadows return to the blade instead of another plane. If a creature tries and fails to use the Banishment spell on the shadows, they may not attempt to do so again for another 24 hours. (Or rather, if they do, all attempts for the next 24 hours automatically fail.)

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.

If a wielder of the blade of languor dies while having a concordance of 4 or greater with the weapon, their spirit is trapped and becomes another shadow haunting the blade, doomed to await another wielder for any hope of physical agency.





If you wish, you as the DM may decide that the possession effect of this weapon represents a single shadow haunting the blade taking hold of the wielder, rather than the collective of all of them at once.

Individual shadows within the blade may have unique agendas, and you may use this as an opportunity to develop stories. For instance, a particular shadow may crave vengeance on a specific person, and use whatever time they possess the wielder for to try and achieve this end.