Black Blade

"The Black Blade is one of the most dangerous relics that can be unearthed, for they contain the souls of the most dangerous individuals in history." - Unknown

Black Blades are named for the first known instance of the phenomenon, Kylvar's Black Blade. The exact conditions for creating a black blade are unknown within the world, though there is speculation by scholars. Black Blades hold the soul of a deceased individual. They are bound to it through traumatic means, rendering them unable to leave the object. Because of this, they are able to exert great influence over those who come into contact with their container. The strength of the captured soul varies by virtue of their experience and how long they are in contact with the holder. 
As they exert their power over the living, they understand their abilities better and begin to refine their methods. Black Blades are capable of whispering to their holder, inducing hallucinations, and outright possession. They rely upon guile and manipulation, though some less refined individuals may try to jump straight to overpowering their holder. Their ultimate goal varies between individuals, with most wishing to live again, even if it means an innocent party takes their place in the Blade. However, the Blades who's strongest emotion is to protect or mentor will forego the possession aspects and instead aim to teach and pass down their skills to those who hold their container. However, that does not mean the holder is safe. A blade's feelings and opinions can still be swayed.
What is thought to be the creation method: Black blades are thought to be created by a strong malicious desire for revenge. The soul of the creator feels such rage and hatred that they are unable to ascend to cross from the living plane to the afterlife. Their soul lingers within the object to manipulate the unwary into aiding them in their quest for revenge or in general exact misery and anguish upon the living. Some are said to even attempt to take over the body of the unwitting holder.
How a Black Blade is Created: Scholars are not far off on the creation of a Black Blade. The soul becomes bound to an object by strong emotions and resistance to death. Revenge, regret, a desire to protect, love, and so on can fuel the creation of a Black Blade. The soul often embeds within an object of familiarity and comfort when a Black Blade is created, however, greater emotions such as resentment or fear can drive it to bond to an object on the executioner. The object the soul binds to is most often a weapon with all Black Blades being created in combat and through violent means.