Nightmare (ナイトメア, Naitomea) is the main antagonist of the Soul series. He is the incarnation of the sword, Soul Edge. Nightmare has been present in one form or another in most games after Soulcalibur, appearing in Soulcalibur II, Soulcalibur III, Soulcalibur III: Arcade Edition, Soulcalibur Legends (as an unplayable boss), Soulcalibur IV, Soulcalibur: Broken Destiny, Soulcalibur V and Soulcalibur: Lost Swords. The Nightmare appearing in Soulcalibur V and Soulcalibur: Lost Swords is a new being. Soul Edge possessed the body of a swordsman and plots behind the scenes under the alias "Graf Dumas" as he plans to resurrect the full power of Soul Edge.
Nightmare has been portrayed in the games as a violent and ruthless creature, constantly in search of souls to restore Soul Edge. He looks down on humans, considering them "pathetic" and "foolish", having no mercy for others, even to those who serve him, as shown in his Tales of Souls path where he considers killing his loyal servant Tira.
Even though Nightmare originally debuted in Soulcalibur, there was a "prototype" of his character in 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, in which he is consumed by Soul Edge. This early version was very different from the current Nightmare, having crimson-red armor and a more flesh-like design with no mutated Nightmare Arm. He has no story nor ending, and only used Siegfried's Soul Edge without effects. Oddly, he uses the same voice actor as Siegfried.
In Soulcalibur, the concept was expanded and turned into an official character, though with changes to the design. His red armour was changed to a dark blue armor and its design turned more metallic instead of fleshy. This was also the first appearance of the Nightmare Arm. Siegfried's body was slowly being mutated by the sword's terrible power, causing his right arm to morph into a hideous claw. Despite the Nightmare Arm only having three large fingers, it didn't seem to hinder his combat abilities at all. He now wields his own version of Soul Edge, sporting a single eye on its blade, something that would become the trademark of the various versions of the weapon, and the creations made from it, such as Charade.
Nightmare was a starting character whose style resembled Siegfried's former moveset. Siegfried was later added to the cast as a time-release character and inherited the same style with minor changes. Nightmare's profile on the game explained how Siegfried obtained Soul Edge after a fierce fight against its former owner, the dread pirate Cervantes de Leon and the fiery demon that spawned within it afterward, Inferno. After that, the sword spoke to him, promising the resurrection of his dead father if he helped it gather souls. Unaware that it was a trick to restore the sword's power, Siegfried agreed and started a killing spree which eventually made him weak to Soul Edge's will. Nightmare was born soon after.
Nightmare's incarnation in Soulcalibur II remains mostly the same, with minor upgrades to his fighting style, mainly on his stance. As his and other character's profiles explain, he formed a group with Astaroth, Isabella "Ivy" Valentine and Aeon Calcos (the Lizardman) in an attempt to speed the gathering of souls and restoration of the sword's power. But during the ritual to restore Soul Edge, Castle Osthreinsburg was assaulted by Kilik, Maxi, and Chai Xianghua. With his allies either defeated or missing, Nightmare confronted Kilik and Xianghua and was ultimately defeated, later falling into the collapsing void where they fought. After waking up, Siegfried, apparently free, regained his senses and tried to distance himself from the world to atone for the sins he committed as Nightmare. But the sword slowly regained control over the young man and eventually Nightmare resurfaced to start another massacre on Europe. In this game, Nightmare is a starting character, while Siegfried turned into a third costume option for him. In his second costume option, it can be seen that most of the right side of Siegfried's chest has been corrupted in a similar manner to his arm, and his right eye has changed to a golden color.