Black reavers are virtually indestructible beings in the Rolemaster source books, who are said to be more dangerous than anything other than a deity. It would take a being of such power and knowledge of the black arts to make such a thing. They are composed of a very powerful demonic entity, such as an Ordainer or other demigod, possessing the physical form of a higher undead such as a lich. They have the obedience of the undead, but the will to destruction of the demonic. In itself this would make them immune to the elements, non-holy, and unmagical weapons. If they are struck down they will resurrect themselves, but their power is from beyond the pale, so they are not vulnerable from having a phylactery.
Reavers wear black full plate with a horned helmet and an enormous battleaxe, which makes them considerably dangerous and resistant to magic. These properties make them almost invincible in practice, requiring divine intervention to be stopped. When a black reaver was released from a box at the tower in Danjirland, it sought to destroy the soul of a king who was on the other side of the Gates of Oblivion. It ripped open a tear in reality to pull the soul out, which was sealed from the other side by the guardian Eissa (Lorminstra). She aided adventurers in its destruction, which required a specially consecrated blade to be aimed into a gap in its armor, and only after its power was severed from the Pales.
In the very rare instance someone has the knowledge and ability to create such a monstrosity, they are somewhat akin to golems in that they are incapable of independent action outside of guarding or destroying. However, they are quite intelligent, only single minded. Black reavers have been known to walk across the floors of oceans to pursue thieves, pulling themselves up onto ships and slaughtering all. They will pursue their objective forever, unless they are destroyed. When they are tasked with the destruction of a target they are much like black stalkers or shadow assassins, or the dark assassin as we know it, relentlessly seeking out their victim across all time and space until achieving their annihilation.
Behind the Scenes
The Vvrael made a black knight that was clearly based on the black reaver concept. It hunted down Daephron Illian through time over thousands of years, slaying the Ashrim royal mage Malaphor and also possibly his assistant Tindal. It was weak by dark reaver standards and was destroyed by adventurers in the Elven Village near Danjirland. While this was an homage to an earlier storyline, black reavers are primarily an I.C.E. Age concept and likely do not exist even in Shadow World. The "black knight" was probably an allusion to the Black Knight in "Monty Python and the Holy Grail", given that the knight had to be searched multiple times before decaying. More generally, the Black Knight in Arthurian legend is the first husband of the Lady of the Fountain, reflected by the bower in Castle Anwyn. He is slain by the knight Owain who is also associated with St. Patrick's Purgatory, which is relevant to the cavern under Castle Anwyn, arguably also representing the Cave of Caerbannog with the killer rabbit.