the same character i have been to believe that they are becuase on SC II nightmare when he had his human form on there he had a scar on his left eye and also has long hair if they are the same character that is going to make me mad to see my favorite character just asexually reproduce

In Soul Blade (the first game), the owner of the original Soul Edge owner was Cervantes. He is defeated by Taki and Sophitia but Siegfried gets a hold of the Soul Edge. He is turned into Nightmare and is the main antagonist in the following Soul Caliburs. However, in Soul Calibur 2, he is able to redeem and free himself from the sword after Soul Edge's spirit's defeat. Apparently it was the end of Nightmare but Zasalamel in Soul Calibur 3, who had followed the Soul Edge progrees from its very creation (Zasalamel is something like 3000 years old) used magic to bind the sword to the memories of his former host and thus, a new Nightmare was born.