Siegfried VS Nightmare

What Wikipedia Says 

Introduced in Soul Edge as Siegfried, he pursued the weapon "Soul Edge" to slay his father's killer, in denial about the fact he himself had killed him by accident during a skirmish. Finding the sword on the body of a defeated Cervantes de Leon, the sword's spirit Inferno (Soulcalibur) used the corpse to protect itself but was defeated. Afterwards the blade forged a deal with Siegfried: if he would harvest human souls for it, it would resurrect his father. Though Siegfried used the it to defeat the evil warlord Barbaros during the events of Soulcalibur Legends, by Soulcalibur the sword dominated him, transforming him into Nightmare.

Allying himself with Astaroth, Ivy (Soulcalibur), and Lizardman in the city of Ostrheinsburg, Nightmare attempted to completely restore Soul Edge. However his allies were attacked and either defeated or abandoned him, and he himself was beaten by Xianghua who wielded Soul Edge's antithesis "Soul Calibur". As a result Soul Edge's control was broken, and realizing now it was he who killed his father and the sins he had committed for the sword, Siegfried hid himself and Soul Edge, fearing retribution from those he had wronged.

By Soulcalibur II, Soul Edge used shards of itself left in Siegfried's body to dominate him completely and use his body to once more cause destruction. Raphael Sorel, a french former-nobleman pursuing the blade for his own scheme, challenged Nightmare and was completely defeated, but managed to pierce Soul Edge's eye with his rapier in one final attack. The sword now in agony lost control over Siegfried, and alongside other warriors Nightmare was pulled into Inferno's dimension, Chaos. Assisting in Inferno's defeat, Nightmare recovered Soul Calibur from its body and impaled Soul Edge's eye upon returning. Both swords sealed, Siegfried abandoned his old armor left with them.

In Soulcalibur III, "Nightmare" was revived in essence by Zasalamel. Created as an avatar for a disembodied Inferno using the original armor as a shell, it pursued and eventually recovered its true body, Soul Edge. Meanwhile Zasalamel manipulated Siegfried, leading him to animated armor, and wielding Soul Calibur it clashed against Soul Edge. At the moment the swords struck a burst of energy was released; Siegfried was mortally wounded, while Soul Edge and its body were pulled into a warped dimension, eventually returning to Ostrheinsburg. Soul Calibur however revived Siegfried, while Inferno restored Soul Edge fully and strengthened its hold over the animated armor, and both renewed their battle against each other in Soulcalibur IV.

Even though the Fire warrior Nightmare originally debuted on "Soulcalibur", there was a prototype of his character in the predecessor, "Soul Edge". In the home version, an extra character named "Siegfried!" was added as an unlockable character. "Siegfried!" was an alternate version of normal Siegfried taken from his no-input ending, where he was consumed by Soul Edge. This early version was much different from the former Nightmare, having crimson-red armor and a more flesh-like design, almost as if he were part of Soul Edge itself. He has no story nor ending, and only used Siegfried's Soul Edge without effects.

In Soulcalibur Legends, which takes place between Soul Edge and Soulcalibur, Nightmare is presented as a boss fight. When Siegfried attempts to reject Soul Edge's power, Inferno emerges and takes on Nightmare's shape to stop him. After the credits, there is a short seqence showing the true Nightmare (or at least his foot) and Soul Edge Male.