The Soul Edge was originally forged as an ordinary blade. However, after years of bloodshed, the sword evolved into a powerful instrument of evil, tainting all those who grasped its power with an overwhelming urge to conquer and destroy. Only one man was able to wield the evil sword Soul Edge without becoming tainted by it, the man who had originally forged the sword in the first place. This man, Algol, sought to use the sword to bring about peace to the world, and was heralded as the "Hero King."
Unfortunately, the sword became so powerful over time that it became truly sentient, and developed a mind that craved power over all things, and desired to create chaos in the world. Soul Edge began to consume the souls of those King Algol slew with it, and every soul made it more and more powerful. Eventually, it was able to reach out into the minds of nearby men and drive them mad with jealousy over the power Algol held in the sword, leading several of these men to attempt to steal the blade. The man who succeeded in this was Algol's own son, Arcturus. Immediately possessed by the overwhelming power of the Soul Edge, Algol's son attacked him, but with great pain and reluctance, was forced to kill his son. Algol then knew that the Soul Edge had to be countered. He took living shards of Soul Edge that had been fragmented off the mighty blade during the clash with Arcturus, and used them to forge the equally sentient sword Soul Calibur, which was dedicated to creating order where Soul Edge was dedicated to chaos. Soul Calibur and Soul Edge were evenly matched, so that the majority of their attention would be spent on trying to destroy each other instead of unleashing their power on the hapless Human race.
Over the course of history, the Soul Calibur, passed on through generations by the guardians of the sword, has been used to shatter the Soul Edge time and time again, though the evil blade has inevitably re-formed from the fragments left behind. After centuries of this process, the sword has taken on an organic appearance, with parts of the blade appearing to be made of meat, with the signature symbol of a living eyeball growing on the side of the weapon.
The form the Soul Edge takes depends on the person holding it; while most Humans are not strong enough to actually wield the blade, their souls immediately consumed upon touching the handle, if a person of sufficient willpower draws the weapon, the living blade will shapeshift into the form of whatever weapon that person is most comfortable using, even if it is in more than one piece. Its size has ranged from that of a small double-edged short sword to an enormous zweihander.
The Soul Calibur series begins with the evil sword in the hands of the pirate Cervantes de Leon. Possessed by the evil sword, Cervantes seeks to slaughter as many men and collect as many fragments of the sword as possible to restore it to its former power.