William Marshall was a wealthy wizard-weaponsmith, with a genuine license to make weapons for the King of Karnivhal and his royal army and navy. Some condemmed him for making his money from weaponry, but he was a nice and honest enough person who stayed away from crime and sedition and loyally served his royal master without complaint, even on occasion willingly donating money to the royal treasury over and above the taxes that he had to pay. Now the King of Karnivhal was a warrior-king, delighting in warfare to the point that he would frequently expose himself to danger, once to the point where the tip of his royal sword was broken off and only his sheild and armour, and a handful of trusted bodyguards, spared his life. So he decided to have magical swords made for him and them to stop such a thing from happening again.
It was in that year that William's sons grew old enough to be conscripted into the royal army, and William, confident that the King would be pleased with him for his many years of faithful service, decided to ask his Majesty to give desk jobs in the rear to his sons. Sadly, he had made a dreadful misjudgement and the angry king announced that they would instead be placed in the very front ranks of whatever battle their regiment ended up fighting in;as officers they were expected to lead from the front as the King himself so often did. Soon after that one of the many small wars, so called because they were limited in size and civilians were not often directly harmed, broke out between Karnivhal and Vallermoore over a trade dispute. The battle that followed was a bloody one, and both of the hacked and slashed bodies of William's sons came back in coffins. Rather then making a fuss, the weaponsmith quietly added an extra spell to the seven magical swords he was making;one for the King, the rest for his royal bodyguards.
The finished weapons were tested on chunks of beef, on old suits of armour,and even against other weapons and they worked well enough even to cut through a discarded Dragon scale. Technicly, these could even slay dragons with a hit in the right place,although getting close enough to make such a hit would be almost impossible. In the next battle the King himself rode into the ranks of his foes, something they had often done before,striking out left and right with their swords. But this time they were to have a shock, for as their blades bit into flesh, so they felt as if they themselves were being cut. He who slashed an arm, felt as if his own arm was being slashed, he who pierced a body as if his own body was being pierced. They dropped their swords,such was the pain they felt, and weaponless were surrounded and cut down, their bodies hacked and slashed just like William's sons had been.
Only one of the swords was recovered offically from the battlefield, and that was buried with the dead King;it is thought the other six were fliched when the battle was over, but it is unlikely they have brought any luck to their new owners.
These swords are indeed deadly weapons, enchanted to parry better, strike harder and cut deeper then an ordinary sword, but when they cut into living flesh, in the short term the sword's owner feels all the pain inflicted on the other person. A strike to the heart may induce a heart attack in the sword's owner, one to the neck, short-term paralysing and blindness, one to a limb or to the torso pain and shock. The effect, if not fatal, is only short term;given a few minutes the sword's user will recover, but time is not something one generally has in a fight, paticulaly one against multiple foes, be it a pub brawl or a full scale military battle, or one where your blow has only hurt your opponent but not killed or rendered him or her hors de combat.
Strikes against the Undead reduce the effect by two thirds, as no living blood runs in the struck one, and there is no effect against metal or things that are inanimate and never were alive. It is possible that for one aware of the downside of this weapon, they can mentally prepare for the pain, which does not last for a long time unlike the pain caused by a real blow, but is in the mind instead. Few are those able to get used to the downside however as the first time it is used is normally fatal for it's owner when he or she drops it because of the short-term shock.