Rare Weapon (Any Sword) , Requires Attunement. 

• Gives a +1 bonus to attack and damage rolls made with this weapon. 
• Deals extra 1d4 necrotic damage.
• Wakeful Evil. Blade of Evil houses a primal manifestation of evil within itself. It cannot speak or communicate directly with its wielder, but it sees through its wielder’s eyes and tries to manipulate them by a system of rewards and punishments.
  • When the user does a purely evil deed that the spirit of the sword likes or is amused by, the wielder will feel a surge of addictive euphoria spark through their body, giving them energy and one of its other possible rewards. If the user does something selfless, good, or anything the sword spirit finds distasteful, the wielder will instead feel pain and exhaustion wash over them, alongside one of the spirit’s other possible punishments  

Here is the list of Rewards and Punishments, the evil inside the sword chooses which one it will provide to the wielder. 

Rewards. 

• Regains 1d6 Hit Points
• Gains 1d12 temporary Hit Points that last until the wielder finishes a short or long rest
• Gains resistance against a single type of damage
• Deals extra 1d4 necrotic damage on attacks made with this weapon for 1 minute
• Gains the effects of the Bless spell 
• Gains advantage on their next saving throw 

Punishments.

• Takes 1d12 psychic damage 
• Gains vulnerability against a single type of damage
• Is affected by the Bane spell
• Gains disadvantage on their next saving throw
• Drops their weapon 
• Loses the ability to speak for 1 hour 
• Is blinded for 1 minute. 

These are only some of the possible Rewards and Punishments, with others being possibly added by the GM depending on the situation and behavior. 

The evil inside the sword can only give out a limited number of Rewards or Punishments per long rest, but that number depends on the GM, but usually the more powerful the Rewards or more severe the Punishment, the less power the evil has.