Soul Edge is the Central antagonist of the Soul Series. It was once an ordinary soul but when it was dripped in blood it turned evil and Inferno was it's soul. Later when used by someone Nightmare is it's body and Charade is it's eye.
Consistently referred to as the "evil sword" throughout the series, Soul Edge has a reputation for devouring souls. The weapon is also known to be a shape-shifter – which form it takes depends on its current owner. Only those with great mental discipline can retain their will while wielding this weapon. In the games, a common trait of selecting Soul Edge as a combative weapon is that it depletes the health of the player whilst giving increased attack power or even draining an opponent's health upon contact. Either way, it is not unusual for the player to lose the battle as a result of wielding the weapon - without the opponent actually landing the finishing blow - thereby living up to its in-game legend of "consuming" its wielder.
When Siegfried took the sword, he created "the Evil Seed", a manifestation of the sword's evil energy that caused disastrous negative effects.
Soul Edge's curse is called malfestation, and is inflicted not only upon those who wield the sword itself, but those who come into contact with its shards. Normally, the Evil Seed transforms the person infected into a demonic monster and feeds off the host's inner sinful deeds of bloodlust, controlling and manipulating their actions, and forcing the user to take the souls of whom they defeat in battle, as it does to Zasalamel when he obtained it, Siegfried-Nightmare, Charade, Night Terror, Aeon Calcos, Necrid, and Cervantes de Leon, from the main series, and Geki and Maki from Soulcalibur Legends, as well as other people mentioned in characters' backstories. The curse's newest victims are Amy, Voldo, Raphael and Sophitia, and its newest allies are Tira, Solnhofen and Johan Dürer. Many people have fragments of the cursed sword inside their body, such as Astaroth and Maxi, and others keep a fragment or fragments of it; some do so merely in order to test their own powers, such as Mitsurugi and Rock.
The first known bearer of Soul Edge was the Hero King, Algol, whose strong will conquered the evil intelligence of the blade. However, his son, Arcturus, was not so strong, and quickly fell under the dark influence of the sword. Jealous of his father's power, he stole Soul Edge and instantly came under the possession of the evil sword. In a desperate battle against the deranged prince, the mighty king defeated Soul Edge, but at the cost of his son's life. Swearing vengeance against the cursed blade, which disappeared after the encounter, he forged from purified fragments of Soul Edge the holy blade Soul Calibur.
The second known bearer of Soul Edge was Zasalamel, during one of his many lifetimes. Trying to break the curse of his eternal reincarnations, Zasalamel obtained the cursed blade, but instead, his soul was possessed by the evil will of the sword, and he turned into the host of the blade until the end of that life. After that, records cease until several hundred years later.
Soul Blade (Soul Edge)
During the 16th century, Soul Edge mysteriously reappeared in a secret antique auction, though it is unknown why it now existed as two blades instead of one. The swords were obtained by an English curio dealer, whose ship was attacked by the dreaded pirate Cervantes de Leon. Possessed by the will of the wicked swords, Cervantes slew the crew of the ship and then slaughtered the population of a Spanish port-town, where he stayed the next twenty-five years, waiting for the birth of the "Child of the Cursed Sword". But then a holy warrior sent by Hephaestus, Sophitia Alexandra, confronted him and was able to destroy the smaller of the two blades. Cervantes, enraged, defeated Sophitia and nearly killed her, but was stopped and killed by a demon huntress named Taki.
It was at that point when Siegfried Schtauffen found the object he had been seeking. When he tried to take the blade lying beside the dead Cervantes, Inferno (called "Soul Edge" in Soul Blade), in an attempt to restore himself, flowed into the corpse and fought Siegfried for the possession of the evil sword. Siegfried won the battle, and when he grasped the hilt of Soul Edge, he was unable to control its power, and released the Evil Seed on the world. Siegfried was further corrupted until he no longer had control over his own body, assuming the role of Nightmare. In the penultimate events of Soulcalibur, he was defeated by Kilik and Xianghua after her treasured Krita-Yuga weapon revealed itself to be Soul Calibur. Xianghua then used Soul Calibur to shatter Soul Edge. However, traces of Soul Edge were still present in Siegfried's body, and he eventually donned the azure armor again.
Soul Calibur II
After succeeding in restoring much of the weakened sword's power, Nightmare was challenged by a swordsman searching for the legendary blade, Raphael Sorel. After a fierce battle, Nightmare stood victorious, but with his last breath, Raphael made a desperate and violent attack and managed to strike his sword into the very center of the cursed blade. Screaming in agony, Nightmare fell back, and within his mind, Siegfried reawakened. After a fierce mental struggle, Siegfried regained his sanity. Shining from the core of Soul Edge, the holy sword Soul Calibur appeared to him, released from its entrapment within the evil blade. Taking Soul Calibur, Siegfried drove it into the cursed blade, piercing its eye and silencing the power of Soul Edge. However, it was not enough to destroy the cursed sword.
Soul Calibur III
Inferno, thrust out of Soul Edge by Soul Calibur, transferred himself into the remnants of Nightmare's armor, but could not manipulate the bodiless husk. It was then that Zasalamel appeared and agreed to help Inferno if he assisted him in finding the two swords of legends, before using his ancient powers to bind Inferno to the armor. At this moment, the evil spirit of Soul Edge became the new Azure Knight and the true form Night Terror. Born from the memory of his old shape, the Phantom Soul Edge is dwarfed by the real cursed sword, but as Nightmare once again began his quest to search for his missing form, this weapon began to grow stronger. The real Soul Edge, silent with its counterpart Soul Calibur, was in the possession of Siegfried Schtauffen, who sought a way to seal the evil sword forever.
Though Soul Edge's power was vastly diminished, there were still many of its fragments scattered throughout the world, expanding Soul Edge's influence far beyond the sword itself. Cervantes de Leon carried another version of the cursed sword, born from fusing several fragments together, as did Taki with one of her blades. Even though the sword was sealed, its fragments still emitted the evil energies of the cursed blade.
Soul Calibur IV
Nightmare and Siegfried eventually meet in Lost Cathedral, and Soul Embrace is finally broken. The two knights clashed, sending a shockwave that ripped both their bodies apart and leveled the Cathedral. Soul Edge and Nightmare fell into a warped dimension, but even then, Soul Edge sought power. Because Nightmare had been given a body by the hatred and evil that dwelled there, the sword and body ended up at Ostrheinsburg when they fell out of the void. Soul Edge stretched its evil roots into the azure armor. The shockwave emitted at the cathedral had spread an evil energy that had made all of the sword's shards active; now Soul Edge called out to them, and they returned both small and great. These included the being Charade and the smaller Soul Edge that had been fused together by Cervantes. These pieces had been scattered by Evil Seed, but now they were being called home. The sword drank it all in, and, for the first time in decades, was mostly whole again. But it was not satisfied, and the sword continued to stretch into Nightmare. Now, the azure knight's body, which had been formed from the lingering hatred and tormented ghost, truly became Inferno.
Finally, drawn to the sword like it was a pulsating beacon, the sword's servants began to gather at Ostrheinsburg. The evil armor was no longer enough to contain the evil power, and so the power and mind of Soul Edge sank its roots into Earth itself, and Ostrheinsburg became a cursed city, a vessel that would consume souls to further strengthen the sword. Now the sword sits and waits for the final moment when it can clash with Soul Calibur again.
Soul Calibur V
Defeated in battle by Soul Calibur, Soul Edge shattered, its fragments passing through a dimensional rift that scattered them everywhere in the world. But the cursed sword's minions tracked down the shards, and reassembled as much of the sword as they could. Eventually, it regained consciousness; using a spare body it had hidden beneath Ostrheinsburg, Soul Edge was reborn.
Soul Edge's new host assumed the name "Graf Dumas", and worked its way into the inner circle of Emperor Rudolf II of the Holy Roman Empire. Undefeated in battle and brimming with knowledge from all corners of the world, Dumas quickly won the Emperor's favor and was granted dominion over the Kingdom of Hungary.
After the new Nightmare is defeated, Tira hands Soul Edge over to Pyrrha and the sword changes shape once again, becoming a smaller short sword. Pyrrha Ω becomes the unholy sword's new host as she promises to take souls for Soul Edge in exchange for the sword never leaving her alone.
When her brother is trapped in crystal, however, Pyrrha overcomes Soul Edge's will and chips away at the crystal, ultimately saving her brother. Together, the siblings seemingly rid the world of both swords, and the essence of both swords flows into Astral Chaos. It is unknown what happened to both Soul Edge and Soul Calibur - if they were truly destroyed or if they formed into Soul Embrace once more. Some shards of Soul Edge, including one broken off during the battle between the two siblings, still remain in the world.