A strange blade that only exists to those whose attention is not focused on it. If you look at it, not only do you not see it, but what you see implies a complete lack of its existence. A hand gripping it is held open; a person it is stabbed into seems to have no wound; thinking about its sheathed weight on your hip makes you feel unencumbered. However, when unnoticed, forgotten, or ignored, it manifests. The appearance of the weapon changes, based upon what one might expect to see; a shadowed shortsword half hidden beneath a rogue's cloak. A solid greatsword strapped to a knight's back. A nondescript arming sword at a traveler's side. The only common trait it has is a peculiar lack of reflection; it is eternally dull, dark, and for lack of a better term, old.
The sword is a beacon, a link, a connection to a greater power. Not a mind, persay, more akin to a manifestation of endless alternatives and blind possibilities, an infinite choir of histories which never happened. All empty, false, hollow--yet desperately desiring form. The chaos that never came to be. It has no thought, nor ability to think or plan, but it does have a singular common feeling. Hatred for that which is, the reality given life, stolen from the uncountable different worlds that would have occured.
Power: The sword is not alive, and possesses no organs, but it can perceive everything within several meters. The closer something is to it, the more it can perceive; if it's in contact with something, it understands that thing fully, in every sense, from its physical form to its thoughts. But objects more distant from it are more difficult, perhaps only their surface texture being detectable, or how they visually look. All that it touches it knows, and all that it knows it understands; the warrior wielding the blade sees his foe, and the blade understands his intent to kill them, even though it cannot see or perceive the foe itself.
Power: The sword is hostile to reality, and its presence unmakes what is around it, little by little. The more it perceives, the less that which it perceives exists. The individual holding it seems beign, uninteresting, one's attention sliding away from them. The noise around it is muted, quiet. Emotions are dulled, life is emptied, the closer the sword. Likewise, when wielded, the holder finds it difficult to focus, or think, their thoughts overwritten by a chaos of false alternatives. The only concrete thing which can exist for their mind is things farther away, and all things that are concrete fills the wielder with disgust. It drives men mad, violent and forgetful, exerting quiet control over their actions and mind.
Power: The sword exerts subtle control over its surroundings. Passively, events occuring unconsciously, automatically, or accidentally are magnified, made easier, and made more destructive; weaker realities force themselves to the front more easily, overcoming what should have been. Actively, sometimes the sword effects substantial change, typically to destroy or replace. It is always seemingly natural, an event easily accepted, but in actuality it has forced a jaunt to an adjacent possibility, killing the old and replacing it with something inevitably worse.
Power: The sword is inevitably forgotten, but always shows up again, wherever one might naturally expect it to be. One day it is dropped in a dungeon, the next a looter finds it on a battlefield, just another warrior's discarded weapon. They take it, absently, not even knowing why, and it finds its way back into the world which lives.