The sword was created at great cost by a rich merchants son that wanted to be an adventurer. The weapon was quickly stolen during one of his first nights out on his own and quickly ended this endeavor.
The weapon to all appearences will appear to be a +5 sword until in battle when a qualified person swings the suddenly heavily unbalanced sword.
The sword gives a +5 bonus to the user as long as he is not trained/qualified to use that weapon. If he is able to use the weapon then they get a -5 bonus in combat.