Cursed magic weapon
An everstriking sword always hits an intended target unless an impossible blow is attempted, such as striking at a target that is out of range of all attacks. This weapon has no bonuses to hit or damage. However, a peculiar effect comes into play when such a sword is used in combat.
If the sword is swung at an opponent and a miss is rolled, then you lose a number of hit points equal to the difference between the failed to hit roll and the target’s AC, making the strike hit, in effect using the your own energy to guide the attack home. Ten hit points worth of damage drained can be “traded” for a level of exhaustion. This energy drain will be felt as a sudden, sharp pain coupled with a feeling of exhaustion.
Any character who uses this sword once in combat will be cursed to always draw the sword in any further combat, and will ignore all other weapons that he carries in favor of this one. If another weapon is used, all attack rolls with it are made with disadvantage.