The Fatal Sword

Full Item Description
You have discovered a blade, hidden beneath layer upon layer of mouldering sackcloth in the back of this storehouse where an ancient wall collapsed. It is fine sword, a blade fit for a nobleman, simple in make and without ornament, with a straight haft, a thick but impeccably-balanced guard and a shining blade with a slight but noticeable curve which comes to a gleaming point. Despite being buried within the ancient, crumbling brickwork and shrouded in oily rotten wrappings, it shines as if newly forged and polished. When turned blade-on, it can barely be seen. It is probably the most well-balanced sword you have ever held, but there is no doubt in your mind that it is the sharpest.

You have found one of the legendary Fatal Swords of antiquity.

History
In the ancient days, there were created several hundreds of the magical blades known as Fatal Swords. Wielded by many of the warriors now idolized as heroes of the legendary eras, these swords have become legendary in their own right. Though over the ages they were lost and forgotten, some destroyed through supernatural means, the tales of their fateful powers remained, as well as the tales of their creation. Some reports of Fatal Swords (or blades suspected of being Fatal Swords) have surfaced over the years. One was known to have been wielded by Ar Ghan The Head-Taker, King of the Toghan Reaches, in his adventures before his ascent to kingship. The sword known as the Mighty One is suspected of having been one of these blades. It was wielded as an execution tool for the dispatching of the Empire's greatest foes and traitors for centuries by the Imperial Executioners of the Emperor of Xi Kah (it was lost during the sacking of the Imperial City by the Beetle-Faced Shangarr during the Shangarr Wars in that country).

Only the most powerful of the primotologist-smiths of the heroic age created Fatal Swords, and while many extremely fine ancient swords were made that far surpass those of the modern era, only a few hundred Fatal Swords were ever forged. Siu the Swordsmith, of whom legends speak of as the "un-man" master of the Swordsmiths Of The Yazda Wastes, created at least 40, but possibly as many as 70, and is remembered in myth as the finest swordsmith in recorded history (some say greater, even, then the Forge God himself).

Magic/Cursed Properties
The Fatal Swords are called so simply because they are fatal in battle. If the swordsman wielding it can at least handle a blade without cutting himself, the Fatal Sword cannot fail to cut its target. The mystical properties of the Fatal Sword are such that wielding a Fatal Sword against an opponent will always result in the severing of either a limb or a head, more commonly the latter- the blade instinctively seeks not surface cuts or cuttings, but the separation of extremities from the body. The Fatal Sword cuts through armor and shields like butter, and even stone is cut by its strike. It is a non-negotiable property- the wielder of a Fatal Sword cannot forebear from severing a part from the target- thus, the sword should not be used against any who the wielder wishes only to subdue or beat into submission. The Sword's employment will result in death or at the very least incapacitation by removal of limbs. The blade makes a distinctive noise (possibly approximated by the onomatopoetic "snick-snack") as it cuts, the sound of the air itself being parted.

The magic of the Fatal Sword is not imparted by direct enchantment, but by virtue of its creation. The process of creating a Fatal Sword requires a feat which modern swordsmiths consider ludicrously impossible- during forging, the metal of the blade must be folded one-million times.

This folding impacts metal into metal so minutely and so thoroughly that within the blade, the metal realigns into post-physical currents, opening the pleromatic voids within and behind all things. What this means, essentially, is that the blade is made so thoroughly that a part of the essence of a sword, the primordial pre-standing cosmic idea of a sword, is imparted as an echo into the sword. Thus, the Fatal Sword is the closest that nearly any sword can come to being the ideal killing tool.

Notes:
This is for What Makes A Weapon Magic Quest as well as the New Takes On Classic Magic Items submission. It is essentially a re-working of the good ol' vorpal sword to represent the totally awesome badassness of the idea of vorpal sword (a sword that automatically beheads the target? Holy shit!).