Originated from the immortal one. When he defeated the dragon protecting the treasure of Nibelung, the dragon's blood poured over his body and solidified his skin like armor and immortalized this warrior. From then on, he received many nicknames: dragon slayer, the one who slew the dragon, etc. It's said he was finally killed when a spear hit him in a spot where the dragon's blood hadn't reached. I guess no one's perfect.