Nightmare (ナイトメア Naitomea?) is a fictional character designed for the Soul Series of fighting games, making his debut in Soul Calibur. Ever since then, Nightmare has been the main antagonist and owner of the Soul Edge, and thus the objective of most other characters in the story.
Nightmare was created as the alter ego of Soul Blade character Siegfried Schtauffen, created when the latter got hold of Soul Edge and was consumed by the sword's will. Nightmare has been portrayed in the games as a violent and ruthless character, always in search of souls to restore Soul Edge. He looks down on humans, considering them "pathetic", and has no reserves even to those who serve him (as shown in his Tales of Souls path, he considers killing Tira eventually). Nightmare's Tales of Souls also shows that he doesn't need to either eat or drink to survive, since when he reaches the Labyrinth he's not carrying food provisions like the other characters.
Nightmare has been present in all the sequels to Soul Calibur, Soul Calibur II, Soul Calibur III (as an own entity separated from Siegfried, which is why he suffered a big makeover apperance fighting-wise) and Soul Calibur IV, where he recieves yet another visual makeover.