This bow is also laced with powerful energy and when attuned, once per turn a number of times equal to the wielder's proficiency bonus the wielder can add 1d8 force damage to a ranged attack made with this weapon. When this feature is used the wielder can also choose to draw from their own strength to add to the power of the attack, making a constitution saving throw (DC 18), inflicting one level of exhaustion upon themselves if failed, and forcing the target of the attack (max 1, can be chosen for AOE effects) to make a constitution saving throw (DC = 12 + (2 * your constitution modifier)), inflicting two levels of exhaustion upon them on a success and one on a fail. These effects can stack if the wielder is willing to risk it. 

This would be for a rogue archer character.

EDIT: The BBEG the players will face is immune to exhaustion so it is mostly meant for fun with the lesser enemies on the way to the BBEG.

Exhaustion is pretty debilitating, so I don't see a player wanting to use this effect on anything but the biggest enemy. If the biggest enemy is immune... the Rogue's just going to be taking Exhaustion levels and giving none in return. If you want the player to feel a bit stronger than they actually are, then pull the rug out from under them when you reveal the BBEG is immune to their whole thing, then this could probably work. Not sure why you'd want that, though.

Way to OP. 3 lucky shots and every creature that's not immune is done.

I have attempted to balance it more and changed some of the wording:

This bow has 5 charges.

This bow is laced with powerful energy and when attuned, once per turn, the wielder can expend a charge to add 1d8 force damage to a ranged attack made with this weapon. When this feature is used the wielder can also choose expend a secondary charge to draw from their own strength to add to the power of the attack, making a constitution saving throw (DC 18), inflicting one level of exhaustion upon themselves if failed, and forcing the target of the attack (max 1, can be chosen for AOE effects) to make a constitution saving throw (DC = 10 + (2 * your constitution modifier)), inflicting two levels of exhaustion upon them on a fail and one on a success. These effects cannot stack more than twice. This means that even if the target succeeds on the saving throw twice and only suffers two levels of exhaustion they cannot be exhausted more. The maximum effect is four levels of exhaustion on two failed saves. This bow regains 1d4 charges per long rest.

Quote from Aeson_Keel >>

This would be for a rogue archer character.

EDIT: The BBEG the players will face is immune to exhaustion so it is mostly meant for fun with the lesser enemies on the way to the BBEG.

????? A rogue doesn't have proficiency in CON saves which means they have 75-80% chance of failing the save and gaining a point of exhaustion. A rogue is crippled at 3 levels of exhaustion because they cannot sneak attack with disadvantage on their attack roll. So why would they use this? 

Suggested rewrite:

This powerful bow absorbs the strength of those it slays. It holds 3 charges, when you hit with an attack with this bow you can expend 1 charge to deal an additional 3d8 necrotic damage to the target and the target must make a Constitution saving throw (DC 18) on a failure the target gains 1 level of Exhaustion. When you kill a creature with an attack from this bow, it regains 1 charge.

Exhaustion is not a great mechanic for one-shots, as it's only corrected slowly through long rests. It's more meant for campaigns where long-term consequences can be felt properly!

Consider instead Poisoned, with further damage if the target is already poisoned?

I do like the thematic flavor of exhaustion as a way to highlight energy sapping, but when I've used it in magic items I've allowed it to grant one level of exhaustion as long as the target currently has no exhaustion. Obviously not a strong benefit, but I don't need to lock down its use so hard (at high level this could just always be on) and it still has plenty of useful applications.