Soul Edge was an ancient cursed sword in the Soul series of fighting games. Soul Edge's main characteristic is its fusion of metal and flesh.
In Soulcalibur II, many warriors set out to destroy the sword, but almost none prevailed. The sword began to control an evil wizard, who was defeated by Link. To prevent this from happening again, Link set out to destroy it, migrating to the world of the Soul Edge. He managed to destroy a fragment of the sword, ending the evil that threatened Hyrule. According to the game, it was, without doubt, the strongest and most dominating sword Link had ever encountered. It was powerful enough to control the wizard being worlds apart. Link set out on a quest to destroy the sword for good.
As one of Link's unlockable weapons, the Soul Edge takes the form of a sword and shield that is similar to the Master Sword and Hylian Shield, both made from Soul Edge's characteristic fusion of metal and flesh. The version that Link wields appears to be nearly fully organic.
It is notable that although Link and Hyrule were featured in the same story as this Soul Edge, its canonicity with the rest of the Zelda universe is disputed.
The relation between the Soul series and the Zelda series is mainly created to explain the appearance of Link in Soulcalibur II. Since the two series are completely different, except for the pseudo-medieval setting, the relation is not solid and should not be considered as part of the Zelda timeline.