Sieg, obtaining Siegfried's heart, is overwhelmed by the dragon blood during the Great Holy Grail War and fights Fafnir in his dreams. It begins to kill him later, but he uses the Holy Grail to accelerate the process and take on Fafnir's form. He, in Fafnir's form, then takes the Grail to the Reverse Side of the World, where there are no humans for it to effect.
Orleans: Wicked Dragon Hundred Years' WarEdit
Fate/Apocrypha: Inheritance of GloryEdit
Sieg, still in the form of Fafnir, is having difficulties keeping control of the Greater Grail, summoning a spiritual body replica of Ritsuka Fujimaru's consciousness from the Shadow Border to aid in the task. As Fafnir he created a terminal of Sieg to properly interact with Ritsuka.