>>96328201
"No one knows exactly how avatars of vengeance form. Some claim that, when a person with a great need for vengeance dies,
his spirit becomes an avatar of vengeance. This story cannot be true, though, as avatars of vengeance are not undead creatures. Rather, they are beings composed of the raw energy of vengeance and vendetta; they are revenge given physical form.
Avatars of vengeance are rarely encountered naturally,nor are they summoned. They are born of a terrible wrong or bitter emotional state β when an individual seeks vengeance with all of her will, she may inadvertently
bring to life an avatar of vengeance. The spirit has a will of its own β even if the individual who created it remits her hate. The spirit does not stop until it claims vengeance upon the perpetrator of the act that caused its creation. Avatars of vengeance do not speak. Most believe that they understand Common and a few other languages, but they have no interest in communication." Copied from the 'Alliance Player's Guide'
tldr; it's a revenge elemental made of revenge energy, the same way that a Naaru is just a positive energy elemental